Windows on ARM doesn't change much
Windows is not scaling down to smaller devices (smartphones and tablets), ARM is scaling up so that it can run laptops and PCs.
This will affect Intel more than Microsoft. As it stands, ARM for Windows will have very little effect on the current PC ecosystem.
Windows used to run on other chip architectures as well. NT kernel ran on PowerPC and Alpha chips, but x86 eventually made them extinct. The Windows on ARM announcement is no different than that. It's simply Microsoft seeing the writing on the wall and making sure it's software will be able to run on all future chip architectures that might power laptops, desktops, and servers.
