As far I understand one of the main reasons why operating systems running on x86 hardware can be unreliable (apart from MS code) is that there are so many system configurations (different chipsets, soundcards etc). It may work fine for some people (the developer writing the os) yet other users have huge problems. I used to use win98 at work and at home. At work it was mostly reliable (by windows standards) whereas at home It was terribly unstable (there is a very good reason why I got a powerbook to replace them both - and upgraded to win2000 for those essential windows programs).
At present apple controls the basic hardware specification of all machines that run MacOS and can ensure that support exists for it in the OS, imagine if apple had to provide an equally good os that supported everyone's hardware. Without the microsoft "do as we say clout" to wield on the hardware manufacturers Apple would probably produce an operating system that for a large proportion of users would be less stable than windows.
Without all their favourite apps very few people would by such a system (i would imagine most people who would by OSX for intel would have a Mac anyway). All that would happen is that apple would lose out on another hardware sale.
Slightly off topic; I'm new to macs (both as a user and a developer), from my perspective cocoa based apps are better (faster, more reliable, easier to write etc.). Is carbon supposed to be a temporary stop gap to allow fast porting of software, with the intention of moving to cocoa in the long term, Or does there exist a long term future for both api's. I read lots of almost "religious" arguments about whether or not finder should be rewritten in cocoa - would this make any difference.
Opinions vary, but from what I understand there should be very little performance difference, but Cocoa is supposedly easier/quicker to code.
Carbon was intended to be a crossover principle for Mac OS 9 - X, so that the DTP industry, who are notorius for taking their time, are still supported and can migrate in their own time - this goes for anyone else too.
The problem with the finder is not so much that it is a Carbon application, rather, it is a badly written application.
and the points you raise about OSX/windows, are some of the reasons I don't believe we will see OS X on intel.