I basically lump everything you’ve listed under “philosophy”—poorly chosen design goals and no one at the project doing anything about dev behaviour are not technical flaws per se, as the software is functioning as intended and expected.
systemd and init+OpenRC can exist together in the same repository, though—it just means that the repository also needs to contain both init scripts and service files, both of which are trivially small.
As a Gentoo user since 2005, I’ve been able to watch the entire debacle as it’s progressed, and the various efforts required to keep udev and friends working separately from systemd. (Currently, there are 7 Gentoo packages on the “absolutely requires systemd” list—6 optional daemons that I don’t perceive as being very useful, although maybe it’s just me, and one library that I’ve never heard of in any other context. So what’s being done to keep it from taking over is working.)