One positive result from Stadia can already be observed. More developers are starting to implement Vulkan renderer in their games and custom engines (common engines all have Vulkan renderer for a while already). And not only that, they finally are starting seeing Vulkan as the real common API:
Example: https://www.youtube.com/watch?v=VCqut9tr6iA
> We realised that if we did a port to Vulkan, which is a
> renderer that is used on phones and various other stuff,
> we get Mac support, we get Linux support, we get support
> for things like Google Stadia and so on - it was just
> worthwhile to do a Vulkan port. So we started that,
> because it's going to be good for Path of Exile players.
I suppose they mean translation from Vulkan to Metal with MoltenVK when they mention "Mac support". But that line of thinking is going to become more and more widespread now, thanks to Google throwing their weight behind Vulkan for Stadia.
Optimistically, pressure on MS, Sony and Apple will increase to the point where they'll support Vulkan on their walled gardens that now enforce DX, GNM and Metal lock-ins, because developers will demand it. And even if they'll resist, projects like MoltenVK and gfx-rs will help break their lock-in despite such resistance. gfx-rs would need to add Vulkan to GNM translation though.
So while I'm not using Stadia due to its idea being basically DRM on steroids, the positive effects from it also exist.