Again, you are not wrong. This is all clearly not intended. However it has become a challenge to map things like Btrfs subvolumes (when seen from a Btrfs mount) onto POSIX semantics [1].
You are absolutely right that ideally there is an update to the POSIX standard. But things like this take time and it's also not necessarily clear yet what the right path here is going forward. You can consider a lot of what is currently taking place as an experiment to push the envelope.
As for if this is a Linux specific problem I'm not sure. I'm not sufficiently familiar with the situation on other operating systems to know what conversations are taking place there.
ZFS has the same problem, for the same reasons. But it also has additional reasons. The simplest of them is that inode numbers are 64–bit integers but ZFS filesystems can have up to 2¹²⁸ files.