If Google shuts it down, they will likely refund all games purchased. (But probably not any subscription fee)
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.
In my experience, it's difficult to get Google to hold up that warranty. When my Nexus 6P started to bootloop, Google refused to repair the product at all nor refund me the money (I purchased directly from Google and I did have copies of all receipts including pictures of the original package it was delivered in).
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.
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.