This implies either (a) refusing packages that depend on systemd, or (b) patching packages. Which brings us right back to the original problem: who's on the hook for making sure it works?
> 2. Bringing up a prompt/dialog during installation to make a choice of whether or not to use it.
https://www.joelonsoftware.com/2000/04/12/choices/
... okay, I admit I also need to offer a reason why it's okay for Debian to offer the install options that they do offer, while they also refuse to offer this option, and can't really come up with one. The way that Debian allows you to pick a desktop environment at install time, while not allowing you to pick an init system, is a bit of an arbitrary decision.
But the decision being arbitrary isn't a reason to offer every single theoretically possible option at install time. Debian never allowed you to pick an init system at install time before! Why start now?