Having any MMORPG go into maintenance mode kind of defeats the purpose of it being an MMO, and I can't really see it as anything other than an eventual death sentence.*
While this doesn't mean the game is unplayable, and might have a lot of replay value for current players - eventually the population IS going to decline as the masses lose interest (and there goes the community, aka the lifeblood of any MMO). Eventually you'll have a hand full of die hards playing it until everyone forgets about it.
Then there's the looming cloud constantly hanging over, where everyone knows the shutdown notice could come at any time. Sounds kind of dismal.