You can of course make the case that the interfaces provided by systemd are substandard, but so long as you don't have an alternative to offer, it's just talk.
As far as I know, Gnome does currently have fallbacks (with reduced functionality) on non-systemd systems, so it's not the case that they just ignore things. However, I perfectly understand why they would not bother with duplicating code just to support systems that aren't good enough.
EDIT: I'm apparently not able to respond to the reply below me for whatever reason, but... It's been said many times that making systemd portable makes no sense, which is why it provides interfaces. And regarding the interfaces, which part of them, exactly, are not stable? There's a very reasonable interface stability promise, which to my knowledge has held, so far.