Steam is probably the one of the few things to keep pc gaming going in the last few years.
The Indie market has thrived in steams marketplace, As have other game types that would of sold very poor in an open market with little to no distribution.
You have to remember something in the uk at least you can no longer buy pc games in most computer stores.
Outside of the top 10 sellers game does not sell pc games. Same with all the big supermarkets if its not in the top 10 forget it.
Torchlight 1 and 2 would never of made either best seller list but its still one of the best pc titles ive played in a while. Similar thing with Dungeon Defenders. Endless space, Sins of a solar empire list really does go on and on.
Without steam those games would not exist its as simple as that as the publishers off them could not afford to pay the fees that big companies expect.