You can theoretically solve a lot of problems with crappy, multitudinous game stores - easier payments, lighter software. But the real question is: do I trust that I'll be able to continue downloading my purchases for the foreseeable future? I trust that Steam will be around for a very long time. I can download DRM-free installers from GOG. Everyone else? I don't know, I have much less confidence.
Why is a launcher even needed? Isn't it better to provide downloads and payments from their website rather than paying 25% of transactions to Valve?
Managing a hundred different games from a dozen different sources is a pain.
If we lived in some fantasy utopia with standardized identity authentication and easily-verifiable-cryptographically-secure-proof-of-purchase then maybe distributed content distribution could make sense.