> they had bought the music on nearly any other music streaming service, they'd still have access to it easily on any device.
No, if they'd bought it on a wide number of other services, they'd also have had to switch to new services with different terms and features. Play Music isn't the only major store to have shut down over the years.
> Now they have a bunch of random mp3 files that can't easily be played on any device
Other than dedicated streaming devices, and devices that only handle dedicated physical media, pretty much any device that can play music can easily play mp3 files, random or not.
Also, the supported automated migration path (YouTube Music) does, in fact, play songs from the personal library ad-free without a subscription:
“You can play uploaded songs in the background on your mobile phone or on the web, ad-free and offline - even if you are not currently a YouTube Music Premium subscriber.”
https://support.google.com/youtubemusic/thread/52908732?hl=e...
So other than you have to click a button to migrate, and you have to use an app and service with a different name, there's no substantive change here.