So I hope for the people's sake that buy into it that Google won't shut it down (or at least have a way to add your purchases to steam or something), but its a different risk this time.
Even when Google shut down their Assassin's Creed demo of Stadia's technology, which was free, they gave everyone a game key to redeem on Steam at the end, so my guess is that's what would happen in a Stadia shutdown.
The most likely "loss" is from the hardware purchases, though Google has hedged here a bit on the fact that a Chromecast Ultra will still be useful post-Stadia. Even the controller might not go to waste if it works as a generic PC game controller.
They have done that previously with all discontinued products. Even hardware they tend to refund in full if it fails during the warranty and they don't have stock of that model anymore, or if they shut down the servers.
The bots don't care if they decide the warranty doesn't apply.
If Google decides to just not refund the game price or offer ownership in other ways, what are you going to do? You didn't buy a game, you bought a license to stream the game from Google's servers. You don't own anything and Google owes you squat.