Subscription gets you "up to" 4k, right now none of the titles is true 4k, they are upscaled from lower resolution 1080p or 1440p.
Yeah, that part is pretty crazy. I wonder if that is just up to the game developers, or if they were given a limited compute budget and the current 1080p / 1440p base resolutions were the best they could get? Maybe Stadia can't deliver the needed latencies at full native 4k?
Unless I am misunderstanding, the 4k up-res is happening on Google's side, so it's not like they are skimping on bandwidth.
Upscaled on the device you already paid for, yet somehow need a subscription to use the upscaling.
So yeah, if they discontinue Stadia it is an open question what happens to the games you purchased on it. They have avoided questions on it, unsurprisingly.