probably not a huge deal, but bigger implications

Can you point me to a standard Unix API for managing processes? Even before systemd, System V and BSDs had their own things, and they lacked a general API for building supervision trees, especially at a per user or session level.

Your memories of CDE aren't also quite accurate - there was a ton of vendor customization of CDE. Solaris, for instance, had its own automounter that they had CDE integrate into. The other vendors had their own and they were all proprietary and different in their own ways. The current state of affairs with fd.o presenting a single interface for i.e. UDisks or whatever is much better than what we had in the 90s. A lot more complicated than mount(8)...