It's extra work for the devs, I guess, since they can't just plop enemies on the ground waiting to die. They have to code when/how they run in.
It's less than you think. The devs already have multiple fights that use similar enough mechanics that it wouldn't be hard to coopt the 3 existing turret fights (the first 2 fights of Colicoid War Games and the turret wave "boss" in Kaon) for similar fight design. The only difference would be that you wouldn't have a turret for this type of fight.
The only problem I would have with a King of the Hill style FP would be leaving it open ended and providing progressive rewards for continual advancement, in a "Survival mode" style of play. As an FP with a story and a defined ending, I could see it happening or even a completely arbitrary survival mode just to see how long your group can go without getting progressive rewards, but, providing progressive, open ended rewards I don't really see.
The biggest problem with the wave-upon-wave design is that you either don't have enough downtime, which makes it stupidly hard to do said content (especially if the waves are timed and the group doesn't have perfect execution or isn't overgeared) or you have too much downtime and end up with players stand around being bored (generally if the next wave is triggered by the previous wave being wiped out). You can solve both of these problems by having some kind of device that you click to bring on the next wave but that begins to get in the way of suspension of disbelief, especially if you build it around the idea of a continual siege rather than a game (which is how Colicoid War Game pulls it off; Kaon manages it with the excuse that the explosive starts making noises to attract the rakghoul waves).
If I were to design a KotH style FP, I would probably use a tweaked design based upon how WoW did the Hyjal Summit in tBC where there are a series of trash waves followed by discrete boss fights with dialogue and events that advance the relevant story. The problem, of course, is finding a way to have the story make sense for both Imperial and Republic players (you could feasibly have pure mirrors using the same geometry and same basic events, but you end up doubling the work on making the models and animations for the bosses; it's more work, but a fair compromise with those people that want more Empire v. Republic centric PvE content) while still allowing for a specific progression of the storyline.
My basic story would follow a similar vein as is used in most PvP warzones (there's a position that needs to be taken), but instead of playing offense, the players are playing defense (conversely, the dialogue could say they're being sent there as part of an offensive but, in transit, the situation changed and now they're on defense). The start of the story would probably involve some dialogue about an offensive being started to take out a specific lynchpin target on the planet (a fortified location that can hold off a massively overwhelming force with a minute number of people with negligible effort), though I would probably tie the ability to hold and use said location with the help of an individual with a special ability (like a General or Commander that is a tactical genius and knows exactly how to leverage the advantages of his defensive position to hold off forever or a Jedi/Sith that is a master of Battle Meditation, on par with Bastila Shan) or, if you wanted to get pretty medieval about it, until they run out of supplies, which they have in great quantity but are susceptible to sabotage.
The FP would start with a breach occurring and the players running up to reinforce it (the breach occurs in a cut scene but the enemies don't actually make it to the breach to be defended against until the players arrive). There are a few NPCs there to help (since it doesn't make sense to only have the players manning it) and the individual waves are strong enough that appropriately geared players will need the assistance of said NPCs. I would set up the waves such that they're comprised of a single champion or pair of elites and a large-ish number of weaks and standards (the elites or champion are there to force a certain degree of ST focus be present and the eak/standards are there to require some degree of AoE capability; of course, the NPCs would provide some degree of both of these for those groups that aren't exactly of optimal composition, as long as they can be kept alive). The end of the breach would finish with a boss-type coming up, knocking out all of the NPCs, and having to be beaten by the players (who are the only ones capable of standing back up from the massive explosion, Force wave, terrifying Force power, or whatever else would be thematically appropriate). If the players die, they rez and come back to see the boss guy beating on the incapacitated NPC allies (they don't actually lose health, but it provides a logical reason why he doesn't just burst in again).
Next, I would have some kind of event occur where the NPCs discover that an asset they believed had been killed out in the field (a Scout that had been sent out to get allies, a commando raider that was supposed to sabotage or assassinate the enemy commander, a beloved commander that led an ill-fated offensive to try and give his buddies some breathing room). The players, since they're the only ones sufficiently recovered from the assault at the breach, head out to retrieve said asset. It's a couple trash packs heading out to the guy, only to discover that he's either wounded or captured. Upon killing the guards and speaking to him, it takes him a while to either escape his restraints (if a prisoner) or get bandaged enough to be mobile by an NPC medic, whether the medic was already there or came along with you (if injured), whereupon he gets up just in time for the boss type guy to pop up. At that point, I'd have a twisted kind of fight where the guy you just rescued tanks the main boss while adds spawn: the players have to keep the guy they just rescued alive (the healer by healing him and the tank by keeping the adds off of him) while killing the adds and the boss (the boss would be immune to taunts and never drop aggro from the NPC so that the player tank can still use AoE taunts without screwing things up and the boss takes continual damage); functionally, the player tank would act as an offtank for the NPC you're rescuing, keeping adds off of him while the healer keeps all of them alive. If your new buddy dies, the NPC goes berserk and wipes everyone (AoE insta-gibb and you reset to the point where you talk to the NPC as he's getting up/out of his shackles so the boss fight happens straight away).
After that, you kill a couple trash mobs on the way back to your base (the NPC would disappear as he ran ahead of you back so you don't have to escort him, since I've always found those to be annoying) where the guys you left behind have recovered. Whatever it was that the rescued NPC was supposed to be bringing back (Scout brings back intel, Commando brings back some ****** combat skills, beloved commander brings a well needed morale boost), make everything look like it's getting better, until the skies darken and enemy reinforcements arrive (possibly on the tail of some massive air support) in the form of a thematically different elite beat down brigade (chemical and biological weapons, psychotropics, mutation and altered form, etc.; something to make them kind of inhuman and suitably terrifying to the defenders). Therein we repeat much of the events of the first wave setup, only this time the waves all fit the theme of the special elite brigade. The boss of the wave is the head officers of said brigade (the CO, the XO, and potentially a third officer; possibly a Jedi Master and 1-2 Padawans or Sith and 1-2 Apprentices, though I prefer the idea of it being just military). The boss fight would be a culmination of the various mechanics the waves followed the theme of, with each individual in the boss being the master of a specific aspect of it.
After you take care of the elite brigade, you find out that, while you were taking fending them off, a saboteur or a turncoat just screwed you over and you have to rush into the bowels of the fortification to try and stop them. There is no trash, just a quick run, where you discover the saboteur/turncoat. Depending on which side is supposed to be finally victorious according to the story, you either fight the saboteur *before* they succeed in what they were there for (if your faction is supposed to end up winning) or after (if your faction is supposed to lose). If the individual is a saboteur, it would be a stealth based fight with lots of misdirection and vanishing and similar mechanics (I'm thinking Deception Assassin/Infiltration Shadow meets Dirty Fighting Smuggler/Lethality Operative). If it's a turncoat, it would be more military in nature and include the possibility of including adds (the soldier loyal to said turncoat that came with him); it would be *especially* juicy, from a story perspective, if the NPC you rescued is the one that ends up turning, based upon witnessing what the super elite brigade was willing to do to take this place (using the logic of "if they're willing to throw *that* at us, I would rather give up that face it again").
You get your 4 bosses, your wave defense stuff as trash, a decent story, and some nice Repub/Imperial conflict going on. If it's the same location, you can even reuse all the same geometry.