Every Flash/Shockwave browser game is becoming increasingly hard to play. Games for ancient Windows versions can be devilishly hard to get running, and games for PowerPC Macs are all but impossible. Emulation is flaky and incomplete for console games of all kinds. When it comes to something like a music library, "buy don't rent" makes a lot of sense to me, but the lifespan of software and particularly games tends to be finite even when you do own them.
Now, I totally grant that Stadia will probably have 10% the lifespan of Flash or PowerPC architecture. But lots of people avoided a string of ephemeral music-streaming services and finally bought in with Spotify or Google Play Music. Lots of people avoided ebooks, but are finally starting to come around. So even if Stadia looks too short-lived, the second or third big game-streaming service might convince people it'll last as long as any other software does. (Along with picking up all the users who are only interested in one or two AAA titles to begin with.)
I share the rest of your concerns though, so that sort of worries me more. Streaming games may be what finally enables the death of free/independent modding, DRM-cracking, tracker disabling, and offline play after years of battles with publishers trying to push them directly.