> For correctness though, Windows is famous for being insistent on backwards compatibility over timespans measured in decades and that must surely be automated to the hilt.

That hasn't been nearly the same goal for decades now.

For instance, Crysis literally won't run on win10 or later anymore.

On top of that, security bugs aren't the kind of thing you can automate away during a rewrite that no one has the bandwidth to actually review.