I would favor a /var/user/something directory.

The fact that nobody does that is pretty much a consequence of the difficulty of coordinating multiple projects that do not have a common authority, not because it is a bad idea.

Again, what do you prefer about that?

Maybe the reason no one does that is simply that no one shares your preference.

Having a clear separation between actual user data files / documents and stuff like cache for different reasons:

- easier to cleanup/wipe without risking deleting works/personnal files

- backup solution doesn't have to have a town of entries in an ignore/exclude file

- same as above for syncing software

- tier storage separation possibility

- disk space allocation separation depending on data vs volatile stuff

Should that count towards user disk quota?