It will be integrated in Youtube under “every“ gaming video as long as the game will be on Stadia.
That alone should be enough to make it a decent success
Basically. When there's a "Launch Trailer" video on youtube with 4 million views and all those people see a big button saying "Play Now on Stadia" and can pay $60 to be playing the game in literally 10 seconds, that's something. Of course the actual audience will be limited to people with good internet, and in certain locations it might not even work very well regardless of how good your internet is. But these are things that can only become less and less of an issue with time.
People aren't patient and people impulse buy, and Stadia definitely lends itself to that.