Over a long enough period of time, technically correct choices are always orthogonal to choices that are useful to users.

I am sure windows, like X, also made assumptions that were violated with the advent of things like HiDPI, multiple monitors, video streaming, etc,.

Once assumptions are broken, you pretty much have to pile on hack after hack to get around them. This is what windows does (10+ different APIs to do the exact same action A, for many values of A), and that is why it supports 30 years of software. This is technically horrible, but it is what is ultimately useful to users.

However, it is absolutely not fun work for developers, which is why unpaid ones won't do it, and prefer the easier start-from-scratch approach.