Windows and *nix are different platforms. Writing crossplatform code is pretty easy and a solved problem. At least 99.5% of code can be platform agnostic. Only a few tiny bits are special.
My background is video games. Which is well known to be a Windows-first environment. I currently work on robotics. The vast majority of robotics ecosystem code is Unix only.
You know what is extremely useful for teleoperation? Virtual reality. You know what platform doesn’t have a way to do VR currently? Linux. Womp womp sad trombone. Mistakes were made.
It’s really easy and not hard to write crossplatform code. My experience is that Linux devs are by far the most resistant to this. It’s very annoying.
Should you care about Windows? I certainly think so. Linux still doesn’t have a good debugger (no, gdb/lldb) aren’t good. Quite frankly every Linux dev would be more productive if they supported Windows where debuggers exist and are decent. So really they’re just shooting themselves in the foot. IMHO.
There are probably international traders who can tell a similar story with Linux replaced with "the US" and Windows replaced with "Russia", but I wouldn't consider that similar story to be an argument for ending the sanctions on Russia.
However, there is an important difference between the sanctions on Russia and the strategy of the pro-Linux "activist" that started this thread: namely, Windows is so heavily entrenched in the niche of enterprise IT that there is no significant chance of Linux's replacing it in that niche with the result that there is no realistic chance of a positive effect of this activism that might cancel out the negative effect you describe. So, I am tentatively in agreement with you.
When I'm writing software, I'm writing it for me, not anyone else. I use linux. If I decide to release anything I wrote, I am under no obligation, nor am I "ethically compelled" to make it cross platform. I wrote it for me. Use it or not.