I'm all for linux support, but clearly he's more worried about a future with microsoft taking their digital delivery business, not usability.
Or Apple, or Google, or Amazon. Steam is, really, just an app store. And that's becoming part of the platform in the modern world. Frankly I think the sanest option here would be for MS to simply buy Valve and turn them into the "Windows App Store". But I can't imagine that happening given all the internal churn that would be required to eliminate all their existing online purchase junk.
Gabe is a billionaire, and left MS to do his own stuff. And Valve culture sounds incompatible w/ MS culture.