... and there's the unhealthy relation. Task bar should look be responsible for .. the task bar, and the explorer should just be a view into the file system. There is zero sane engineering justification for the two to have any relationship whatsoever.

Isn't that what the"Start folder windows in a separate process" setting is for? More stability at the expense of more memory and less responsiveness. In the days of Windows 95 it was actually beneficial to not do that. Nowadays, not so much, I guess. Although it's been ages that Explorer crashed on me. Not having too many shell extensions probably helps here.

I think that "new process" menu option is a bit of a lie. I use it every time, but I've had supposedly separate Explorer windows (including the task bar) crash simultaneously.

Due to how the whole Windows Shell mechanism works (COM all the way down) there will still be lot of shared state between the supposedly separate processes.

Not as much in Windows 8+ since it seems that the "Windows Shell Experience Host" process started taking over increasingly more of the taskbar duties. It still seems possible for particularly bad Explorer process crashes to crash the Windows Shell Experience Host (presumably due to COM communication channels between them?), but not every Explorer crash affects the Shell Experience Host anymore. (More interesting now is when the Shell Experience Host crashes; Explorer tends to keep working just fine, and unlike before, opening a new Explorer window doesn't guarantee that the Shell Experience Host reboots.)