That's really unfortunate... Linux alternatives like Pinta don't get even close to Paint.NET's quality.
I really wish you would expand the app reach to Linux community. I'm sure the Linux community would be willing to port the app on their own, and likely even contribute code back to the project. I wish it was at least partially open-source so that someone would be able to properly port it.
It seems like you guys gave up on open-source because some trolls decided to break the law and misuse your work and the work of others, and not even give you any credit. That's really unfortunate. But I kind of agree with this SO comment... I think it's even more unfortunate to completely close the doors for anyone to legally and properly port your wonderful app to other platforms; yet someone who wants to reverse-engineer it can likely still do it anyways (assuming his comment is accurate).
Would you please reconsider? Maybe there's some alternative to make it work...
1. You could reconsider making it open-source, at least partially... This would be ideal because you don't have to worry about any official support or learning how to do it yourself. You focus on the Windows version and Linux community takes care of the rest. If you want, you can have a license that, for example, forbids them to sell the modified version, and requires them to link to the donation page (and give credit, obviously).
2. If you happen to be willing to port it yourself and just lack resources, maybe consider crowdfunding? (I don't know how well this works for software, just throwing an idea as a possibility, in case you haven't considered it). With things like Flathub and the Snap Store you can still make it easily discoverable on Linux even if you keep it proprietary.
3. If none of these are options, maybe you can host your code privately (e.g. on Github, etc) and provide access only to someone willing to port it, upon request, and under conditions like not publishing the original code. Then say "ports are welcome" in the project page or something...
