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.
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.)