Irrelevant is the word, unfortunately

That's simply not true.

Each distribution might use their own init but these are still inits - they do that one job, not everything else, and you CAN easily swap them in and out to replace each other - on slack for instance we have a custom sysV init (but with BSD syntax) but that can be swapped out for e.g. runit, or OpenRC relatively easily. Each of these init systems is different but each performs the same role, and can fit in the same spot in the system.

Systemd does not work in that paradigm, you cannot pull out a standard init and replace it with systemd without replacing a lot of unrelated items as well, nor can you replace systemd with a standard init. A large section of the architecture of the system has simply been replaced en masse, by something with a different structure, a fundamentally different design from the mature design we are familiar with.

And of course personal experience is not statistical data, but I dont see you producing statistical data either and I know a great many programs will compile just fine on various linuces and unices with no more effort than ./configure, and I think the first program I ever ran into that broke that expectation was Gnome. Certainly Gnome and systemd/CoreOS is the first major project I remember taking such an openly contemptuous view towards anyone not eating the exact same dog food they prefer.

So the way I see it, we have a great big *nix ecosystem that includes the BSDs and the Linuces and anything else that wants to be compatible, but the bigger players (Redhat and Canonical) do not want to be compatible with it anymore. So what we are moving towards is a three-way schism. Slackware, Gentoo, and OpenBSD at least, along with some hobbyist projects, will continue to be *nix. Redhat and Debian will become CoreOS, a mostly incompatible fork that will continue to consciously move in the direction of less compatibility every release, while Ubuntu will continue to flog their own versions of the same idea creating yet another mostly incompatible fork, also likely to evolve in the direction of less compatible each iteration.

If that's what they really want to do fine, but I prefer *nix and I suspect most of their users would as well, at least if they understood the question.