Gamestop might like a word on that if their PC section at every store I've been to is any indication. Steam, GOG, and I guess Origin if you really want to include it pretty much are the PC market these days. Is it feasible as the sole distribution method of all games right now? Of course not. Is it feasible as the primary distribution method for games though? Plenty of evidence pointing to yes. And since broadband coverage continues to expand in every part of the world, it's only a matter of time before it's perfectly feasible as the only distribution method.
As for solid state drives, they're a non-issue. Most people don't have them because they're expensive, and those that do, aside from having them in a netbook or Macbook Air or something (where you're probably not looking to play modern games anyway) are going to have your standard drives as a storage drive as well they can run games from. Moreover, the price per GB is always falling on SSD's and I'd say it'll be a couple more years before the amount of storage you get for a reasonable price is more than sufficient to handle just about anyone's gaming needs. And with services like Steam and sufficiently fast internet, you don't even need to store your entire library on the drive all of the time anyway. SSD's are really a non-issue as far as digital distribution, and by the time they've proliferated enough to be an issue, they'll have become cheap enough that it still won't matter.