I bought a couple of games on Stadia, and subscribed for a few months here and there to play a few of their free Pro games.
The biggest loss to me personally will be not having the ability to stream The Elder Scrolls Online. Every once in a while I'll get a notion to play the game, but locally it takes up about 200GB - so I usually delete it after a while of not playing it when the notion passes. The last few cycles of this, I've just streamed the game from Stadia where it doesn't take half a day to install. If I end up playing it for more than a week, I'll start downloading it from Steam. Sometimes I'll even keep playing on Stadia while it's downloading, and when it's done I'll log off Stadia and on to my local Steam copy, which is pretty smooth.
Also, similar to Game Pass, their new "play for x minutes free" thing is usually a pretty good way to demo a game without having to buy or install it.
All that being said - overall it won't really be missed. Outside of a few edge case uses for me personally, it doesn't do a whole lot to fill any particular niche, and it never garnered much mainstream appeal.