Times have changed, guys. When cpu chips have 64 cores each running 4 threads, scattering bits of the OS and bits of applications all over tha place dynamically, and virtualizing a dozen OS copies on top of that mess,... is that going to make things more reliable?
Furthermore,
Fewer points of failure must be better than many points. Few security vuls must be better than many, many. The "entire running monitor" could be tiny, and could run on a fully protected, conservatively designed and clocked CPU core. Its MTBF, hardware and software, could be a million hours.
Hardware basically doesn't break any more; software does.
The virtualizing trend is a way to have a single, relatively simple kernal manage multiple unreliable OSs, and kill/restart them as they fail. So why not cut out the middlemen?
John