You say Microsoft aren't bothered with long upgrade cycles, yet Microsoft has been so aggressive to push towards its "1 billion Window 10 installs," that they have done everything from offering it for free (completely robbing them the direct "sales" revenue), to pushing it to consumers without consent (a thing I like to label and will always label as borderline malware-like).
Indeed, rapidly and frequently "updating" your software and changing it can be problematic, and the thing is, Microsoft has been doing that and is committed to do that. "Major updates every 6 months" ring a bell?
I agree that OEM sales was Windows biggest market back in the day, but that's not Microsoft's idea for the future. as Dorsetknob mentioned, MS is aiming to turn Windows into as close to a service as possible. Licensing local software isn't as lucrative as it was back in late 90s and early 2000s (compared to other strategies), this is the age of data monetizing and services renting (and the latter mostly because it leads to the former).
Settling for "Hey, sales are steady, let's just remain the way we are!" is a mentality for a content, small time, neighborhood shop, not a multi-billions dollar worth corporations! Growth is what drives those entities.