You're conflicting things. GNOME doesn't rely on systemd, it wants an API (a few actually, but AFAIK the only difficult one is the bits from logind). The entire API is hard to implement. The parts that GNOME uses maybe can be implemented by someone else. Maybe not. Someone from GNOME tried to do that for FreeBSD but gave up after a while (problem was the lack of interest, difficulty combined with not being a *BSD person). KDE also will use a small part of logind in case you use libinput as well as Wayland. But again it doesn't rely on systemd, it wants a dbus API.

If someone makes something else, go for it. But API wise it is best to stay the same.