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.
For example, not all distros ship and use systemd-resolved by default, to choose from your list.
But, who is counting?
It has some really nice tools and features that Grub lacks (i.e. it has tooling for checking the state of things like secure boot and analysing the security risks of your boot configuration), but every mainstream Linux OS I've used still relies on tools like Grub to boot.
I have some gripes with systemd-boot's limitations (notably, the insistence on an unthemed, white-on-black menu system that's not exactly enticing to Linux newcomers) but it's hard to deny its merits. Grub is tied together with a spider web of scripts calling each other, loading modules, generating code that is then executed again, and one mistake in one script can cause the bootloader config four scripts down the line to fail, leaving the system unbootable; the concise configuration file for systemd-boot makes for a much better bootloader configuration system in my opinion.
For many people Linux is not an academic exercise. It is a tool they want to use. The don't care to use a different network manager or resolver.
And that is exactly the same as Windows. There is one solution across the whole system and it works together because it is written by the same people and tested together.
I already laid the basic foundation and have the kernel loading into memory and booting. Next step is to get the memory map and pass that along. It's BIOS only for the moment; EFI support will come later, along with other architectures. (PowerPC is next.)