> Now, the "you can't delete things because then the disk space is unreachable" argument makes plenty of sense for, say, FAT, a filesystem from the 1980s.

Unix semantics were IIRC in place as far back as v7 (1979), possibly earlier - granted, a PDP disk from that time was bigger (~10-100MB) than the corresponding PC disk from a few years later (~1-10MB), but an appeal to technological progress in this particular example case is a moot point.