And I think this bears repeating, the first case where /usr is on a separate partition and stuff breaks during boot if it's not mounted is the _current_ case, not the case being introduced by the proposed change. There has been some effort over the previous years to audit the boot process and move binaries from /usr to / to fis this when the issue is detected but this has been scattershot and incomplete.
The proposal to just move everything to /usr and make sure it is always mounted directly after / _fixes_ this problem once and for all. The fact that this also fixes another set of thorny cases is just gravy, the fact that this has been successfully done on other big UNIX systems is a great endorsement. This doesnt have anything to do with systemd trying to eat your baby, It just exposed more pre-existing cases that were broken in the current setup.