systemd is a monolith in practice, despite what its advocates like to claim. You can't run just a part of it under another init without doing extra patch-up work (see elogind). Whereas you can run just one GNU utility on top of, or even alongside, someone else's implementation of that or other utilities (for instance, in parallel with a rust implementation that isn't quite ready for the big time yet).

systemd also won't work on anything except Linux. Older solutions also worked on BSD. That matters to some people.

And the issue was never just Poettering's employers. He had a bad reputation in parts of the Linux community long before systemd—he was also the main force behind pulseaudio, which was shipped long before it was ready for actual use in the real world and remained in a semi-broken state for quite a long time afterwards. And he often comes across as personally obnoxious. Nothing like telling someone "I'm not interested in fixing your issues with my project" (except less politely) to get them to adopt your code.