But the kernel is monolithic, [...]
Wrong. Linux kernel has modular architecture, but monolithic design. The precise opposite of the systemd.
I know that the uninitiated see the kernel as a big black box. But actually Linux very well structured and highly hierarchical.
systemd can monitor dæmons (only if they use the systemd watchdog facilities) and automatically restart them if they stop talking.
The question here is about the case when systemd itself "stops talking".
Some time ago that would have been theoretical question, but actually there were already such bugs in systemd where it was simply stuck because the daemons which compromise it couldn't understand with each other. This is precisely the consequence of monolithic architecture in combination with modular design: the singular logic is spread across many "modules" (the *-systemd binaries) and when the modules do not agree with each other, things go south pretty quickly.