I don't get this guys, what's so wrong with backwards compatibility? Was Win95/98 somehow worse than it could be by allowing you to restart in DOS?
The biggest problem with 95/98 is that much of the hardware is accessed directly, which gives a lot more potential for instability.
WinNT/2000/XP virtualizes pretty much all the hardware access, which is one of the main reasons for the added stability. Under Win95 a poorly written application could mess with some of your hardware devices directly, and kick the feet out from under the entire OS. under the newer editions, that won't happen anymore, hence the increased stability/reliability.