Why would people go elsewhere? Steam does game distribution, and it does it *well*.
Uplay, origins, etc. are all obvious cash grabs trying to pad the bottom line rather than attempts to improve customer experience and satisfaction. Did any of the alternative publisher distribution platforms pass on the savings to consumers? Hell no.
I understand and agree that in principle having a single monopolistic monolith dominate the sector is not a good thing, however benevolent it may be (for now). But up till now Valve have shown no evil. They don't freeze accounts with the infamy of Paypal, or sell user data like FB, or peddle microtransactions-paytowin in games they develop/publish (AFAIK, I don't play DOTA or CSGO).
They have the goodwill of their consumers. So why would the average PC gamer go elsewhere? Valve seems to actually care about games and gamers. And the mountains of cash they rake in too, of course.
Even if Valve did turn evil PC gamers tend to be a reactive zealous bunch. Didn't GTA5s rating avalanche after they removed mod support? Can you imagine the furor if Valve tried to introduce a subscription fee?
GOG and Humble are perhaps the only real non-evil alternatives.