As a software developer also working on desktop software, I can relate to the sentiments Serif have shared in the past. Partly. Some thoughts;
We develop CAD software, and while we only publicly release a Windows version, we maintain a MacOS, Linux and Windows version internally. We're able to do a lot of this because we have developed a lot of our CAD technology internally. Our application is written in C++ using Qt, which does make cross-platform deployment a bit easier (but it's still not easy!).
Why do it when our customers all run Windows? Well, we have three main developers, one of us prefers to use Linux daily, one prefers Windows and another prefers MacOS. Importantly for C++, this means that we use three different compilers for all of our code daily - Clang (MacOS), MSVC (Windows) and GCC (Linux). Each one throws slightly different warnings and has slightly different behaviour where not covered by the spec (so-called 'Undefined Behaviour'). So we've convinced ourselves that running multiple operating systems internally actually allows us to ship more stable code.
We found the hardest part if we were to support cross-platform versions is actually deployment. If I had to take a wild bet, I'd guess that Serif are able to generate a Linux build inside their office inside a week. But that's not the hard part. What do you do? AppImage? Flatpak? They all have their upsides and downsides, and are invariably more complicated than the options on MacOS and Windows. That's not the end of it. An important part of desktop software deployment is management of software crashes. In the industry, this is best handled by a separate crash-reporting mechanism that can be in-process or out-of-process. Using google breakpad is a great solution for this, however out-of-process crash reporting (more desirable for some reasons I'm not going to unpack here) is supported by different underlying technologies on each platform (particularly on MacOS due to Gatekeeper), which means separate support. You could just use the built-in tools on Windows and MacOS (particularly nice on the App Store), but then you're back to needing to maintain a bespoke solution for linux.
Nonetheless, my DAW software of choice <moderated> works flawlessly on Linux, Windows and MacOS and I use it on all three. I've never used software so stable. Often when I have troubles at work with some of our cloud Linux stuff, I think to those guys! (And I think Cockos is only 3 developers!)
As you may have guessed, I'm the clown of the three on Linux. And I would love if my licenses to the Affinity suite worked there obviously!