On these forums I hear many people say things such as:
- blizzar only evar said that wow would go up to level 100 then it would end!
- when we fight sargaras then wow will end :O~~
- thar is only enouf content for 2 more expacs max then wow will end
- WoW uses an old engine is looking dated so it must end soon
No, no, no, no. What exactly is it you think Blizzard will do? Run out of ideas and close the servers? Do you think they are incapable of upgrading their engine?
"Sorry all 10 million of our subscribers, many of you paying $8.99 per month, we've run out of ideas and are incompetent, go play something else".
Blizzard will continue to invent storyline (like MoP), and continue to make major changes to the game well into the future, even if they "only" have 1 million subscribers left.
There is nothing stopping them moving the entire game to a new engine and carrying across all of their art assets. Or even making a major change such as scrapping the levelling system. Or even making the entire thing more like Guild Wars 2.
WoW will continue on far into the future. I would be surprised if it isn't still going on in 15 years time.
UO is still going strong and look how old that is.