[number6]

Right, they changed that 15 years ago.

Splitting would make them more interchangeable and stable - crashing one part does not tears the whole stack down.

What is the benefit of putting all the different functionality in one application?

[awilfox]

Explorer was never the WM; you could kill explorer.exe off in both 9x and NT systems without affecting window management.

I don't remember how it worked in 9x, but NT used CSRSS.EXE for window management until NT 4 moved GDI into the kernel. DWM handles it in Vista and newer.