It’s presumably impossible for Bashar Assad to “win” the civil war at this point, if “win” means “regain control over the entire country in relative peace and stability, and reestablish diplomatic relations with the rest of the world.” The rebellion has gone on too long, the sides have become too entrenched, and Assad has been responsible for too many war crimes for that to be a realistic outcome. But he is certainly in no danger of losing the war and being forcibly removed from power, either. The rebels are fighting amongst themselves, which could be a bad thing for Assad in the long run (if ISIS is driven out then it’s possible that Western aid could start flowing to the rebels again), but which is obviously good for Assad in the short run. In fact, the ideological composition of the rebel coalition, at least insofar as it is perceived in Western media, has been so radicalized that it’s clear that Western powers no longer view Assad’s continued rule as the worst possible outcome. Assad isn’t stupid, so he knows what happens to dictators like him once they lose power, so he’s got no reason to think that giving up would mean a nice, luxurious exile (and at this point even if the rebels didn’t kill him he’s got a standing reservation at The Hague).
So, all that said, why would Assad participate fully in any Geneva talks if those Western powers are going to insist that he step aside, either immediately or eventually, as part of any deal? Imguess they could promise him immunity from warmcrimes prosecution, but that’s not much of a carrot for a guy who’s not in any immediate danger of losing power.
It should also be noted that the “moderate” Syrian National Coalition, the mostly-expat rebel group that we support but that none of the other rebels especially like, is in danger of tearing itself apart due to internal dissention about whether or not it should participate in these talks. None of the other rebel groups, so far as I know, are going to participate. So who is Assad supposed to be negotiating with?