probably not a huge deal, but bigger implications

My memory is that the "API" we had so far, based on "config files, /dev entries and reasonably portable commands like mount, ps, etc." has been, as you put it, "so much complicated, brittle and resource hog".

There are reasons every desktop environment in existence comes with its own "session manager" (one is that before systemd, there was nothing portable or uniform to manage the user session at all, and no, a 4,000-line bashrc ain't it). In my older days, playing audio used to be related to a wonderfully refreshing experience of using lsof to find who was holding up the sound hardware this time. Using removable media was invariably connected with a cornucopia of permission errors, then begrudgingly dropping to a root shell.

Sure, you could count on the commands to be "reasonably portable", for some values of "reasonable", but there are all sorts of hooks and crooks when it comes to actually using them all together and making assumptions about things. Look at the old sysvinit scripts for popular services — you can see how they grew in response to edge cases being discovered. They are very far from being simple. Knowing and understanding them could sure bring joy to someone fond of collecting arcane bits of knowledge.