So... let me get this straight. Stealing a page from many popular casual games, you in effect put a limit on how much XP a person can mill at one given stretch. In a game you need to buy, and pay a monthly fee for. Furthermore, you justify this by suggesting people swap around to other classes during this time. Okay. Ignoring the fact that unless you have a lot of classes that'll still turn off the hardcore players in a huge way, this means any long-running character will work... EXACTLY like any other long-running character, being capable of all the same things, in all the same ways. Any and all uniqueness vanishes.
Which kills part of the point of these kinds of games re: the social element. You are always the tank, you are always the healer, etc, etc. Your gear is specialized for that, your selection of abilities or talents or feats or whatnot are taken with that goal in mind. To quote an old Penny Arcade strip about Phantasy Star Online, I just want to hit stuff with my sword. I want to be able to focus on my given role in a combat, not be stuck playing an entire poorly-specialized RPG squad in one lone body.
To sum up, this is a bad idea from a fiscal perspective, this is a bad idea from an escapism perspective, this is a bad idea from a fun-seeking perspective... it's just a bad idea. Period.