We actually don't know this. It's not a guarantee that an extremely profitable company doesn't want to exit the market and will exist "as is" in perpetuity. The fact that they've put themselves into a position to be bought out is its own strong signal they are considering exit strategies.
Activision's own history is full of being the bucket other huge publishers (messily) exited into. (Vivendi, being a huge example, and itself carrying the combined exited weight of Sierra, Davidson, and Knowledge Associates. Blizzard being another strange example.)
Companies generally have very strong "will to live" certainly, but boards get tired and investors get cranky, and sometimes companies "need" to make an exit.
It's possible Activision remains a large publisher if the Microsoft buy out fails. It is also just as possible that vultures see what is left of Activision as a dead man walking and start to pick at it immediately and strip mine it for parts. (Atari, Infogrames, THQ, …) It is also just as possible that Activision will be between a rock and a hard place and forced to take the next best offered deal. (Would you be just as upset if Activision became Sony exclusive? Or if Activision became just another branch of EA?)
The alternatives right now include a lot more than just the binary of "Microsoft buys Activision" and "Activision remains in its current state", especially because "Activision tries to sell to Microsoft" is a strong signal that the current state either isn't sustainable or isn't as strongly as interest to the board/investors as people might think.
"Activision continues to exist as-is" is a bad reason to block the purchase. Activision may not survive either way. ("Activision may not survive either way" of course isn't a good reason to be pro-buyout, of course, either. I'd think that goes without saying, but sometimes here on HN the obvious needs to also be reminded in conversations full of polarized opinions. I'm not explicitly pro- or anti-buyout here, I'm just reminding everyone that companies sometimes die and there's no "natural state" here to argue about.)
(I don't have much to add to the speculation about exclusives and will they/won't they Microsoft keeping their bargain on cross-platform initiatives. I think that debate is going to keep going in circles.)