The main argument I hear is about portability. Systemd integrates deeply with Linux and isn’t available on BSDs or other Unix-like systems. This in itself wouldn’t be an issue but major desktop software started to integrate systemd locking out BSD users. A recent example is KDE Plasma Login Manager (KDE itself still works).
Providing systemd-less option is something very in line with Gentoo Linux portability goals so I’d take a look at what they do if you’re interested.