The issue with Windows support for tools like Pixelmator is that a great many of its features are wrappers around OS level image manipulation libraries that come with macOS/iOS - Windows doesn't have anything like the rich image manipulation libraries built in that macOS has, so to get feature parity would very likely involve building from scratch a ton of the stuff they didn't have to do on macOS. The Pixelmator developers have said this before in their own support forums too when question of Windows/Linux support is asked.

This is partly why we often see new image editor apps only hit macOS/iOS sometimes, especially if its from a smaller development team.

> This rules out use in most of the professional world

I don't agree with this; I have never worked at a company the design team weren't all on Macs, regardless of company size. Sure it rules out some professional use but I doubt it's even a majority. The output image file assets can be shared with any OS etc etc so not like it stops collaboration either.