I seriously don't see the "free" 1080p streaming staying free. Not once Google sees that of the few that do use Stadia are going to just stick with the free model. Once they realize how much streaming is actually going to cost them, they'll put a price on it. It may only be $5, but any cost is going to turn off some. I also can see them jacking up the 4K price to $15. Google is not going into this business to take any losses or just make a small profit.
And there are plenty of gamers who will be turned off by having absolutely no access to your games if the internet goes down. Even with digital buyers, at least they know the game is there on their HDD and they can still play it without internet. People who actually buy Stadia games are going to get real pissed if there is a sudden loss of internet, either on their end or server side, and it drops them out of a game.