Do you really think the lvl 50 flashpoints and operations is everything there is going to be? With a level cap of 55? Logic is a wise thing, my friend. Sure, you can use boxershorts for 5 year old kids when you're a grown up, but will they fit? No. Simple logic, man. Also, I would only assume that class stories would continue past level 50, wouldn't you? Doesn't make sense to abandon what makes the game amazing, at the very least right now. I doubt EA would like to lose more subs than they already have, so discontinuing class stories is out of the question.
Your assumption would be wrong it has been confirmed by the devs
there is no class story in Makeb. So yes logic is a fine thing but people aren't necessarily logical - including game devs.
As for FP and OPs what most people currently twitchy about is not that they will leave current content as lvl 50 they will instead regear current content to require lvl 55 and those without the expansion are SoL