But then, you're not talking about the bulk of games, since those won't be designed for Stadia. Even the port will be a port from a non-Stadia platform, merely to get it to work on Stadia, most games will not be reworking their games for Stadia. I get the potential for games with specific Stadia builds but in several years I imagine 90%+ will still not be using that. Most will simply be the same, performance wise as what's out there. And since performance, for us, is irrelevant, because Stadia runs it, not our devices those kind of improvements don't really help the consumer, as long as they're staying true to the tin, that you get 4k60fps performance.
Heck, on the idea that they might put out some 'improved' version of the game on Stadia, there's been cases in the past where games have been purposely 'limited/downgraded' so as to not be too different from the lesser quality versions available on the consoles. As long as a game is multiplatform there's not likely to be a huge will to make the Stadia version a somehow improved version compared to the rest of them unless there's something in it for the developers (ie, Google pays them).