The debian installer should first loudly warn that having a separated / and /usr may break things in the future but not forbid it. With that in place,This is not true: you just need to use an initramfs.
Ok, so it should warn that this setup will soon require to use an initramfs. I just pointed out that this implies a slower boot and consumes times nearly at each package install (plus initrd tools seldom guess right everything that should be in the initrd to function).
"I have always done this in a different way" is not a valid use case, sorry.
Same for your proposal : nothing really sound except systemd wants it...
And again, this is not related to supporting a merged /usr scheme.
I think I gave you other reasons for not using an initrd that you have not answered (other did also in the discussion) and other gave you reasons to have /usr separated and not mounted at boot time that you have not answered either.
-- eric