"The Macintosh ecosystem was moved from PowerPC to x86 successfully, why shouldn't such a move work for Windows?"
Much. Much. Different. For one, Macs are already a closed platform. Much smaller market share. Remember, Intel was trying to migrate away from x86 back before enterprising AMD came up with x64 to extend the life of the architecture.
Since the PC is so open, theres nothing stopping a competitor (intel/amd) from rolling up its sleeves and creating a kickass x86-64 chip which would likely dissuade a lot of adventurous early ARM adopters from even bothering to try and make the transition. Apple made the decision to ditch PPC, and that was that, as they control the hardware and OS of their platform solely.
Also, IBM/Motorola's lost revenue would pale in comparison to Intel/AMD if the PC platform completely ditched the x86/64 architecture. So, while IBM/Motorola didnt put a whole lot of effort into meeting Apple's demands with chip design to keep macs on PPC, you could bet your life Intel/AMD would fight to keep x86-64 on top at this point.