On embedded machines it's usually not interesting to split off /usr anyway. And systems without /usr split off can boot without initrd just fine.
Note that the same people who are behind the /usr merge have actually been working on making Linux work with a read-only / out-of-the-box, for use in embedded and stateless systems (We are mostly there, but not entirely). We have the embedded use of Linux and systemd firmly on our map, but initrd-less systems with /usr split off are definitely of little usefulness in our eyes, since they are useless for embedded and pointless on bigger setups.