Classic has a pretty clear shelf life. Since it seems most don't want Classic+(why? I have no idea), they'll just turn the service off after Wotlk Classic has been out for awhile. If you want to constantly relive the glory days, then I suggest you invent a time machine.
They've said they'd keep the servers on no matter how few people log on to classic. The business reality of it is that they will absolutely terminate the service if they are no longer getting a return. How long do people really want to spend in Wotlk? How many want classic to progress into the current game, going into Cata, Panda, Wod etc...? The subsequent expansions after Wrath are not universally loved like the first 3 installments of WoW are. It would soon become excruciatingly redundant to watch the game evolve (again) into the thing that made most people wish for Classic in the first place. No, since people think they don't want Classic+, it will end at Wrath and that will be that.
Personally, I think they will realize they'll have to do Classic+, but for everyone who thinks they don't want that, this thread is for you. Your character has 3-4 years of life left in it.