It’s designed to be down. Because the implementation is an “after the fact” (sort of a hack to be honest) implementation, a VM “running” doesn’t depend on it at all. The key is actually the hypervisor itself. Vcenter is merely an abstract out of band orchestrator that attempts to move things around on top of a pseudo-cluster. So, I stand behind the idea that ESXi is the critical piece where the VM runs and Vcenter, while it is very very interesting, it’s not the critical dependent piece.
Now, if what is being monitored is the capability of moving hosts around (for example), certainly, that is where Vcenter comes in. Or anything else “orchestrator” related.
Exaggeration ahead, it’s like saying a programmer can’t program without a high resolution twin set of monitors. Some might say, “that’s true”.