dgs: "In both cases, the users concerned were moving for reasons of features, support and supportability, not underlying technical aspects of the operating system concerned."
I did say "a crucial reason" rather than "the crucial reason", but agreed: if you have an operating system based on DOS which then has a ton of support libraries for doing GUIs, threading and the like, then the availability of such stuff, no matter how nastily it's done, overcomes the limitations of the underlying platform; this makes the platform easier for people to develop for and delivers a potentially better user experience as more and nicer stuff is made for the platform. I bet Netscape 1.x and successors could have run on RISC OS, too, had there been library support to make the porting bearable.
Another factor is price, availability and hardware support - moving off NeXT or RISC OS hardware onto something moderately inexpensive almost always involved generic Intel-based PC kit, and until Microsoft made the big push into the "consumer market" with their NT-based operating systems, there wasn't that much available kit for that market running NT. That said, NT4 and successors have been available on laptops for years, but I suppose the "consumer market" demanded pretty icons rather than the improved stability that those operating systems offered.
markee174: "Apple had to abandon its update to Mac OS 9 (Rhapsody) and effectively put the MAC OS on top of NextSteps Darwin core."
A pragmatic business decision and, for once, a sensible one from Steve Jobs (disregarding the confusion around Rhapsody, which was derived from NeXTStep as well - you're thinking of Copland which probably deserves a vapourware status comparable to Galileo). I still don't get the claim that "nothing has really changed", though, when all evidence suggests that everything apart from RISC OS has.