IMO, Steam is primarily responsible for not just saving PC gaming, but helping it flourish again.
Prior to Steam, pubs were starting to just give up on the platform due to a) rampant piracy, and b) nobody was buying games due to 100 different permutations of borderline insane copy protection.
Steam gave publishers a reliable, easy DRM method and games distribution platform tied up in one.
While people like to crap on Origin and Uplay, the fact of the matter is that Steam showed them how it was done. Dealing with Origin and Uplay is far better than what both companies tried prior.
As a result of this, the selection of PC games we have to choose from has never been better. Even genres that had been dead for years like the dungeon crawler (Legend of Grimrock, Might and Magic X Legacy) are seeing a resurgence because of these digital distribution platforms. Those games would have never seen a retail shelf, but are enjoying very good sales to incentivise other pubs/devs to create games too niche to sell any other way than digitally.
Steam does not get enough credit for the enormous uptick in quality PC games coming out, IMO.