Isn’t systemd that open standard though?
No. It doesn’t support BSD, just for starters, even though most of the established desktop environments originally ran there as well as on Linux. So by definition, anything that relies on systemd can’t “support as many systems and platforms as possible”. And to my knowledge, no actual standards document defining protocols and interfaces has ever been published (although I admit, as an OpenRC user, I don’t pay much attention), meaning that the interface can change without warning. If systemd works for you, that’s fine, but don’t try to build it up into something it isn’t.
That isn’t quite true. There have been several proprietary implementations for non-Linux systems—Apple’s XQuartz was still being maintained as of a couple of years ago, although I don’t know about its current status. Standards documents exist, and anyone can code to them.