Because it diminishes a player's accomplishment. While I have not unlocked the rocket boost myself, I have grinded out armor, schematics, and various mods/enchancements. If EA is going to offer an alternative way of gaining these items, the cost should be a little more in-synch with the efforts players who bought / unlocked it with credits put into it. I think that's only fair.
I'm not arguing that EA shouldn't change credit costs, drop rates, ect as they deem needed for the evolving game, I'm just saying lets make sure the CC cost matches the alternative method. Otherwise the system encourages pay-to-win.
What exactly in legacy perks give you pay to win? Rocket boots are a convenience pure and simple. Nothing in the cash shop or legacy gives a player a massive boost that suggest pay to win. It does not diminish your accomplishment that you ground out 4mil creds. The real accomplishment was that you had the patience to save for it. What price would in cartel coins do you feel would be sufficient? $10 in real money $20 perhaps. It is a nice perk to have yes but it is in no way a slap to your face. You still made the sacrifice and that still means something. However with the credit cap CC is the only way to get the f2p players to spend money. They need something to spend it on and I think $5 is pretty reasonable. Besides most of the other global unlocks are similar in price.
Taking your scenario of it needs to be in line for time invested vs cost (assuming it takes 2 hours of dailies per day at a gain of 250k per day) I would need to work at a real job for the equivalent of 18 hours. Now given that minimum wage (lets not worry about higher earners because that makes this even worse) is not an even number lets say $8/hr for just neat math sake, I would have to spend $144 in real money for the comparison to credits earned per hour. There are many variables here because I may make more in credits than you do per hour or even less which makes the real world comparison higher or lower but then this is just about an average as 250k is reasonable doing dallies. Now we are only doing some rough math here but you get the point being that it would make the game cost prohibitive and very out of sync.