The issue raised here is a legitimate concern, but ultimately not one that I think will bear out.
The truth is there are already more free games on the PC than you can reasonably play in your life time. Between emulators, abandonia, flash games, piracy and more, we still pay for good games with good experiences.
There is also the discussion of value. A casual game on the phone is something I play in spurts, and not worth much in terms of my money and time. It's something I play in a waiting room, or bathroom, or while I'm waiting for the wife to do something, etc.
The traditional PC game, on the other hand, is something I set aside time to do. It's not something I want to be interrupted in the middle of with ads, or pay walls. This isn't 100% of the case for everyone, but I feel that the PC experience is a completely different one from the casual market we saw with phones.
The danger comes from the AAA developers who've (mostly) ceded their place as leaders of gaming, and now chase the latest fad or graphics upgrade. They may see this as a fad, and a way to push little guys out of the market, but will ultimately not find that to be the case. The indi scene is actually pushing gaming right now. It feels a little (emphasis little) like I felt in the 80's with gaming. Like anyone can do it, and the ideas are wild and out there and there is no one saying no. Some of it's crap, some of it's awesome, but the trip between the two is one I'm enjoying again.