One point you are missing out there is that most users don’t install jack nor have such sophisticated requirements, they just want audio to work and be reliable in simple situations (watch videos, play music, make calls through skype/hangout/…).
We are in a known situation today which is working well enough to cover those cases and there is no compelling reason to switch now (from your arguments, 1- is a specialized setup, people who go install jack could as well install pipewire sound server if they wish, 2- is there any such software in the Ubuntu archive yet? 3- is basically back to the first point, there is no conflict on a default installation)
We will eventually want to switch but at this point pulseaudio has been around for longer and is better tested, pipewire is still new and needs to stabilize (reading comments online from fedora users quite some people have latency issues or sound artifacts and are switching back to pulseaudio to workaround those problems). It’s not the sort of change we want to do in a LTS cycle.