As explained elsewhere in this thread, using initramfs is still not mandatory in debian and I personally have more managed debian system installed without it than with it. And the fact that /usr break may require user interaction is not really handled by initramfsand may cause dangling sysmlink in /Remember that / and /usr don't have to reside on the same partition with the usrmerge proposal: they only have to be both available post-initramfs. The initramfs already takes care to mount /usr (for the systemd case as initscripts needs updates for sysvinit as was said elsewhere). So no repartitioning should be required on upgrades.
-- eric