I think Microsoft will let Windows slowly die over the years. I am certain that at the strategy level, they have already accepted that their time as a device platform vendor will not last. Windows will be on life support for a while, as MS slowly corrals its massive client base onto its SaaS platforms, before it becomes a relic of the past. Beyond that point, the historical x86 PC-compatible platform lineage will either die with it, or be fully overtaken by Desktop Linux whereupon it will slowly lose ground to non-x86 proprietary platforms over the years.

The average end user will be using some sort of Tivoized device, which will be running a closed-source fork of an open-source kernel, with state-of-the-art trusted computing modules making sure nobody can run any binaries that weren't digitally signed and distributed through an "app store" owned by the device vendor and from which they get something like a 25% cut of all sales.

In other words, everything will be a PlayStation, and Microsoft will be selling their SaaS services to enterprise users through those. That is my prediction.