Though in name it's not a new OS, Windows 8, 10 are new experiences for anyone who uses them since Windows 7.

I think you're right about momentum, MS isn't able to come out of it's momentum of the old into the new.

For example, Apple transitions to a new OS with confidence and takes the ecosystem with it in a blink of an eye because they are trying control their momentum instead of being driven by it.

Edit: Windows can maintain backward compatibility with its ecosystem if they did it well, like Apple has done.