I’m a little surprised that a service with 40M subscribers doesn’t seem to consider that users might do this.
After switching regions I also switched back to Spotify and I haven't bothered using it since. I just went back to check in the Apple Music after reading this—surprise surprise, all my playlists and songs are gone too.
In general the country switching experience seems pretty sub par: if you had just purchased a year long app subscription (I believe some apps can offer that), your options for changing country would either be to forfeit your purchase by calling up to terminate the subscription, or wait an entire year for it to expire...
If my Mac routinely deleted files from the filesystem Apple would be working overtime to fix it, but with Apple Music data loss seems to be the norm.
I looked into switching to Spotify recently, but one of the first songs I looked for wasn't available which turned me off (the song is available on Apple Music).
How else would they tie it? Anything else used as the key would still cause you to lose your library if you changed it.
Besides, was it really lost? You can switch your old ID to a new ID and keep your library. Was it lost or merely waiting for you to link it to the new ID?