[AstroDogCatcher]

In a cloud setting, I guess having user /home dirs on separate volumes from the root disk and having them relocatable is a reasonable analogue. The real issue is that much like the pen drive thing, absolutely no-one actually does this.

[hasperdi]

No one? Some people were cared enough about this use case to push it through, so I don't think it's no one. Minority maybe, but not no one.

[gorgoiler]

What place do these niche solutions have in the main project?

In the old days you would write a little script to set up the mapping for your USB drive, once it is plugged in. Linux provides the UID mapping feature, udev gives you a place hook your script, and you glue it together the way you want.

If your solution was particularly high quality it might get included as a package in the OS. Someone else might write a different solution that gets packaged alongside yours. Maybe they scratch the same itch in two different ways? Maybe theirs is just better?

Systemd on the other hand is judge, jury and package maintainer. What they decide upon is what gets included. They are the cathedral, where once there was a bazaar.