I think "basic" in this case means "integral to the system". Many people don't want to replace the init system, nor the desktop display system, nor an authentication system (e.g. PAM).
The more systems something sits under the more places there are for it to cause a problem because it doesn't work quite how the upper components expect. At a bare minimum, you'll often run into problems with the included package management system (and how it may reload systems for security purposes after patching) if you don't take care.
nano sits on top of everything, and the few things that might rely on it use a very standard and not very extensible interface (the EDITOR environment var). It's not really equivalent at all.
I'm not tring to convince you, i'm sharing my pov. Systemd is here because of pov like this, and people not liking it end up with it anyway because they fail to aknowledge others pov.
I doubt that supervisor is not installed because of not solving something it evidently does not want to solve, but that argument at least looks valid to me. Just in case you are not aware, https://github.com/Supervisor/supervisor/issues/122 lists two solutions (wait_for_it.sh and a supervisor plugin that adds dependency resolution).