I'm not so sure about that. Microsoft certainly knew about the Bay Trail, Clover Trail, as well as the outlines of Haswell, and probably Broadwell when it made the decision to support ARM. Intel's roadmaps go out nearly a decade (though they obviously get more speculative the farther out you go) because Intel not only has to design and manufacture the CPUs, they have to build the fab lines that will be used to manufacture the CPUs, and the supporting chips for those CPU's, and the motherboards for those chips. I think the decision to add ARM support was made because Windows NT needed it. It's prosaic, and doesn't make a good story, but I think it has the virtue of being supported by history.
Microsoft has always made sure that Windows NT was supported on at least two different CPU's. One of them has always been x86, but there has always been another CPU that was supported, all the way back to the dawn of NT. Windows NT was originally developed on the MIPS R3000 CPU (used by Silicon Graphics) and then ported to Intel - just to make absolutely sure that no x86-isms infected the codebase. When MIPS declined in importance, the next release supported the DEC Alpha machines. When DEC went under, the very next release ran on Intel Itanium machines. Itanium flopped in the marketplace, but Microsoft kept supporting it with NT until Windows 8, when Itanium support was dropped. And lo and behold, Windows 8 supports the ARM CPU. ARM is the only other CPU architecture that is important at the moment, and the logical choice for the "other" CPU architecture. Maybe in another 5 years they'll drop ARM and add support for the Mill or something.
Apple does the same thing BTW. The NeXTStep OS ran on PPC, x86, HP Prism, and MC86k. After they were acquired/took over Apple, all consumers ever saw was the PowerPC version but internally Apple made sure it still ran on x86 (which was the only other CPU that was still around). So their switch to x86 wasn't terribly surprising if you knew the history of the OS, nor the port of the kernel to ARM for the iPod/iPhone/iPad lines.