To that I'd just go "sure!", but I'm just a guy. The first thing that jumps out to me, though, is the question of who pays for the doing of it and the testing of it? From what I know (and I've been learning a lot today, it's been productive!), it seems like you're likely to undertake a Devuan level of effort to do it with Debian, and your patchset is going to be so broad as to effectively fork the core anyway. Is there value, then, in tagging along with that major distro? What do you get, then, if you're the second-class citizen? I'm reminded of stuff like Kubuntu, which (and I say this as a long-time KDE fan) has historically limped along with only moderate love--and that's a DE, it's a lot easier to handle DE differences than core init differences, service management differences, etc. (We have the XDG standards and the like, after all. Though I guess, in a certain light, systemd's trying to be XDG-for-the-core, which attacks the question we're looking at directly, yeah?)
The other thing that jumps out at me is just that this hypothetical is mostly kicking the can down the road. If Fedora suddenly decided to support multiple init systems I think it's hard to envision a future where the happy path wouldn't remain systemd; it's what the core folks think is best, otherwise they wouldn't use it now, and even with additional support the lion's share of attention would go to that happy path. To that end I think that any sort of init choice in those distros just becomes "so we'll support it for now, but the set of third party stuff that supports it is going to continue to shrink, so I hope you have an off-ramp in the future."
Between these two issues it seems like separate distros that have a firm commitment to not-systemd end up being necessary anyway and an ecosystem of not-systemd stuff will need to compete for value on those merits. What am I missing?