If different choices were available for init, DNS resolver, service control manager, volume manager, etc... we would adversely contribute to the schizo distro landscape the people holding the money bags are actively trying to get away from.
With systemd it's an all-or-nothing deal. You get the good with the bad, but all distros shit the bed in the same, deterministic way.
Not even Windows does this. There is no "systemd" equivalent. Yes, Windows ships as a single OS—as do the BSDs—but all the components were developed separately.
If all they wanted was a service control manager, there were many (better) options already in existence they could have used.