Actually, the gigantic release of Funcom's Age of Conan makes it clear that the upper limit for MMO subscribers is really unknown. Plus, MMO's do taper off over time, and those players tend to look for greener pastures. WoW is king right now, but this KotOR Online is probably still years away from release, and who knows what the market will be by then?
You have quite a few MMO's going strong right now, and by strong I mean over 100k subscribers:
WoW (of course)
LotR Online
EQ2
AoC (impressive start, not sure on lifespan)
EVE
FFXI
Lineage 2
I'm probably forgetting a few, and there are some that manage to cling on to enough people to stay in business: The original EQ, Asheron's Call, Ultima Online, D&D Online, Vanguard, SW: Galaxies (I think this one is ending)...
Source please.