Well from this POV it's kinda openrc's problem if it doesn't. What about trying to run any part of the Openrc software suite on an Upstart system? The question why would anyone sane want to is rhetorical tho...
Why obsessing over whether systemd is monolithic and in what measure anyway? There certainly ARE optional systemd parts. So it's correct to say it's not entirely monolithic.
The rest of the system for example chrony, sysklogd, cron, etc run fine on upstart systems, because they aren't tied to systemd and are fully modular.
It's okay to be a monolith, that doesn't make it inherently bad or anything, but we should be honest about it, and it does come with some tradeoffs.
People experience "blue screens" and kernel panics and such pretty often.