I think this is a terrible idea for one reason: there would be very little content for people to do post-50/51 through 55 (or the next level cap -5 ) if the Classic and current Level 55 Ops were constantly being re-engineered to be higher and higher levels.
i will say first that there is plenty of content for players to participate in from 50-55, and i would also point out how difficult it is to find 8-16 people of level 50 to even run this content in the first place, so there's a lot of 55s, and the content is rather trivialized.
but really, they don't have to be mutually exclusive. lv50 ops and lv55 ops. i imagine that nothing was done in the initial design to make this easy to implement though, and that's probably why we just see adjustments to the operation as a whole (like tfb going to 55 and retuned for that level), but it would be nice to give us more content to do at endgame.
as it is now, there's 3 raids for level 50, and only 2 for 55, and only one of them is relatively new. the other is about a year old now. granted, yes, that is about to change in about a month, but endgame really should have more of this stuff to do.
i think that they did well with the flashpoints, and i agree with Marius_Xerxes that we should get some nightmare mods of lv50 HMs for 55
i would also love some nightmare modes of the current 55 HMs that drop actual endgame gear, but are just as challenging as an operation in terms of mechanics.