perhaps running out of inodes could be taken "more seriously"?

Even XFS can run out of inodes. Inode numbers are mapped to blocks by a very simple mapping. It is roughly like every i-th block can have inodes. The possible inodes are just numbered starting by one. Once these blocks are filled (with inodes or data), XFS cannot create new inodes. Changing the mapping would change every single inode number.

We had once the following problem after growing a filesystem. Standard was at that time to only use 32-bit inode numbers. After growing the filesystem the 32-bit inode numbers where all in the already filled lower part of the filesystem.(*) Thus no new inodes could be created. Took a while to find that one only having the meaningful message "No space left on device.". Luckily it was a 64-bit system. Thus, we could just switch to 64-bit inode numbers. The other solution would have been to recreate the filesystem, not the quickest solution with a 56 TB filesystem.

That said the circumstances under which XFS runs out of inodes are very rare. So it would be very important to have meaningful error messages, to notice that one of these rare circumstances just happened.

(*) On fs creation XFS usually chooses the number i to be such that all possible inodes have 32-bit numbers. After growing this condition was not satisfied any more, as this number cannot be changed. On 32-bit systems, one would need to set this number i manually at fs creation time, if one wants to have the possibility to grow the filesystem.