I agree about Stadia in general. I wouldn't want it to become too dominant, especially since it's like DRM on steroids - you can't back up anything from there. In general, I don't think regular gaming stores that sell rather than rent games are going anywhere. I.e. GOG, Steam and others are going to stay around. As long as Stadia won't be pushing exclusives, things will be good, since some will always prefer to run games locally.
[1] https://www.defectivebydesign.org/
[2] https://www.defectivebydesign.org/what_is_drm_digital_restri...
Since I assume Sony & Microsoft know this, and they aren't countering with a VR push but instead trying to match them with streaming, I suspect they judge the market either isn't there yet, or may never be there (on any business-realistic time scale). Or they just can't resist the DRM and lockin.
All one needs is a screen with support (e.g. smartTV, chromecast, steam box), internet, and controllers. Classic game systems might be needed for another 5-10 years. Controllers & form factors matter (e.g. Switch, VR), but the "power" of PCs & game systems are no longer differentiators.