Re: "now that it's well into Debian, Ubuntu really has no choice"
"But it should be an independent system. "
No, no it shouldn't. It's about the most fundamental thing in the dist aside from the kernel. Why do you expect it to be independent? Everything else needs to start somehow and this is the thing that does it.
Many things will depend on systemd especially if the packages for the dist are done correctly. But packaging is the dist's problem. If your dist breaks because of the packaging, it's not the fault of systemd any more than the kernel failing because some module it wants to load isn't there because of bad packaging.