"Compatibility with Windows programs" is a massive undertaking in the first place, as evidenced by the huge amount of development effort that has gone into Wine without quite reaching 100% bug-for-bug compatibility. (The level of compatibility they've achieved is truly impressive but it's really difficult to get to 100% for a large existing base of arbitrary applications.)
Reliable real-world compatibility requires not only implementing Windows APIs as documented (or reverse-engineered) but also discovering and conforming to quirks, undocumented features, and permissive interpretations of the specs or even outright bugs in Windows that some applications have either intentionally or unintentionally ended up relying on over the years.
I don't know if modern apps would tend to be better engineered to actually follow the spec and to only build on features as documented but for example older Windows games were sometimes notorious for being quite finicky.
And of course if the goal is a full-scale independent OS rather than a compatibility layer on top of an existing one, there's the whole "operating system" part to implement as well.
Reliable real-world compatibility requires not only implementing Windows APIs as documented (or reverse-engineered) but also discovering and conforming to quirks, undocumented features, and permissive interpretations of the specs or even outright bugs in Windows that some applications have either intentionally or unintentionally ended up relying on over the years.
I don't know if modern apps would tend to be better engineered to actually follow the spec and to only build on features as documented but for example older Windows games were sometimes notorious for being quite finicky.
And of course if the goal is a full-scale independent OS rather than a compatibility layer on top of an existing one, there's the whole "operating system" part to implement as well.