The Humble Indie Bundle is already going this route: Overgrowth: Alpha preorder page
(notice the widget is a Humble Bundle widget that the actual sale goes through, and the game gets added to your Humble Bundle library).
I like Steam being curated (and currently, the Greenlight stuff feels less and less curated), because then if my (in-development) game stands out as *higher quality*, being on Steam can get me sales and publicity. But if tons of stuff is on Steam, regardless of quality, then it just becomes as crowded as the smartphone app stores.
You always have to do marketing, but it was previously motivating to know that even just getting on Steam was a guarantee of some level of success.
It's becoming more and more a race-to-the-bottom of pricing (like the app stores), and overcrowded clutter of cheap games with the real gems buried underneath. This is beneficial for stores and publishers (Valve, app stores, consoles), but not for developers. It benefits consumers by very low prices, but also makes it harder to hear about good games unless they get major media attention.
This is still much better than things were ten years ago, but it'd be worse than things were even just three years ago, IMO.
I'm guessing the general direction Valve is wanting to take Steam, is to have people use a Steam "widget" on their own websites to sell their games directly to users, but with users having a central game "library" things get added to.