Two big things here:
#1 - At some point you have to assume that a company's future actions will likely be similar to their past actions. We know EA will continue to not even think twice about dicking over players or developers if there's even a remote possibility of it meaning they can show 0.021% more profit for their next fiscal quarter. On the other hand though Steam's actions over the past years have shown themselves to be far above petty profiteering. They've shown a primary interest in players and games over and above all. This changing is of course a possibility at some point -perhaps Gabe steps down and is replaced by satan in a suit- but that leads to #2.
#2 - Steam is thriving because of their good will. The reason they are an attractive distribution outlet is not just because of having a trillion players, but because they treat their developers extremely well. A standard ballpark Steam revenue share is 70/30 in the developer's favor. Flip those numbers and that's what a favorable revenue share used to look like! Still does for most publisher-tied console developers, in fact. Steam doesn't have a monopoly in the sense that Windows is a monopoly. Developers have to target windows since that's where all the users are and if you release a *nix game, windows players (which is the vast majority) cannot run it. Developers do not have to target Steam - they choose to. A player with Steam can just as easily run a game without Steam. Steam even provides tie-in functionality to allow them to add non-Steam games to their Steam library should they just like the interface. There's no coercion whatsoever. Compare that to Origin which, in typical EA fashion, is attempting to compete through coercion as opposed to good will. The point being is that if Steam loses their appeal - their good will, they lose their developers and they lose their games. They basically end up as another EA trying to coerce players into coming back of "DotA3 - exclusively on Steam" and it will flounder, as Origin is. They wouldn't die or kill PC gaming. They'd just become destroy Steam as a desirable destination for new games and be replaced. The process of replacement would be painful and slow, but it'd happen and we'd all move on.