If you define "care" as in, "random updates cause enough instability that they make alternate OS's favorable for accomplishing the same tasks," I'd cite Windows autoupdate as fairly hard evidence that people don't care.

I imagine MS has crunched the numbers at some point, and decided that the amount of lost work and interruptions due to forcible updates (especially contrasted with the benefits of updates) is not causing a significant number of people to ditch their OS for a competitor.

Competitor? Windows is still >85% of desktop use or thereabouts - you can buy an expensive mac or you can go to hell and try out linux; neither is really an alternative for many so they just accept what happens.

So why do they bother adding new features to Windows?

I'll answer my question: Companies make decisions on the margin. They don't care that they have 85% of the market. They want that next percentage point.

The users who might switch to or from competitors are precisely the "swing voters" they have to consider.