It's not just Microsoft.
Apple are also reliant on Intel investing heavy R&D into CPUs that can crunch lots of data without hammering the power. Their MacBook range is built on the same "Core M" CPU as used in Microsoft's own Surface 3.
If Intel are effectively giving up on all this, then they're placing Apple in a similar situation to the one they found themselves in back in 2003, when they had similar problems with PowerPC.
Now here's the thing: AMD have licenses for both x64 and ARM. They even have chips that include both on the same die, though the ARM part is usually one of the smaller ones. It doesn't take a genius to see how they could reverse this, with the ARM part doing the heavy lifting while a couple of x64 cores are retained to provide legacy compatibility when plugged into a dock.
Coupled with AMD's graphics IP and this makes AMD a rather tempting purchase for Microsoft, who then have everything they need to design and build their own hardware, from phone to Xbox, effectively in-house, allowing a managed migration away from Intel's legacy architecture to something less monumentally shite.
Apple already have their own ARM-centric chip design teams, and an ARM version of OS X is doubtless already up and running in their labs. They also have a strong track record in switching architectures (680x0 >> PowerPC >> x86/x64), and no worries about legacy software, so switching to ARM isn't going to be a huge deal. Remember, Intel aren't giving up on the Xeon end of the market, so the Mac Pros should be fine for a while yet, though with GPUs already doing so much of the grunt work these days, it may matter less and less whether even these run on Intel or ARM.
As for Office: this used to run on a number of different architectures and platforms, and has even made the transition from 680x0 to PowerPC in its long lifetime. The key problem isn't porting to a new CPU architecture, which would require spectacular levels of incompetence to cock up in this day and age, but the fact that it's a very old app designed in an era when WIMP and CLIs were the only user interface games in town.
Furthermore – and this is something too many people forget – MS Office is itself a major development platform. There are entire industries that have built up around integrating Office into their own custom solutions. When people talk about Open/LibreOffice, they conveniently forget all this; some businesses have invested 6-7 figure sums into customisation that cannot be trivially ported to another office tool platform.
Which means Microsoft need to find a way to support all this stuff on anything from a tiny 4" touch-screen smartphone to a 55" Surface Hub, by way of a conventional laptop. This is a big ask, but it's likely they realised they had to bite this bullet a while ago. They just weren't expecting it to be shot into their face so much sooner than expected, nor by such an old friend.
While I don't think Microsoft are even remotely perfect, they do currently sell the only mainstream alternative to the myriad thinly-disguised flavours of Unix out there. Given how often the Commentariat prattle hypocritically on about "choice" and "freedom", I, for one, would rather Windows stay. Without it, the only "choice" is Unix, and the only "freedom" is to choose one slightly different flavour of Unix over another. There is no way that ends well.