IMO, the is a monumental underestimation of the man-hours put into make Windows as compatible as it is, even ignoring the fact that Microsoft's engineers are/were highly skilled.

I like the ReactOS idea, but I think it will take decades to be compatible enough - and I'm even talking about enough.

If the objective was to run obsolete software, I personally had to distribute resources, I'd just do assign them to Wine. It's surely much more productive to make software compatible on a per-case basis, rather than writing a whole operating system with the objective of being compatible.