But if practicality is the concern they'd use Windows or Linux.

Then why work for 18 years to make make a fully fledged OS if in the end the expectation is that it does nothing? For an OS practicality is pretty much always a concern, if not necessarily the top one. Otherwise it's relegated to literal "toy" status. An OS without any practical apps might as well be a GUI made in Flash. Looks nice, does nothing.

If I was developing software on Linux, which distro should I choose?

Can I write a native GUI program that is guaranteed to work on all Linux distros, just as I can develop native software on Windows and target all Windows users?

Yes, but there is less stuff you can assume to already exist on a Linux desktop environment than on a Windows one, so you either need to bundle those with your application or have them as requirements.

Sure, see for example: https://www.flatpak.org/

I am waiting for the next big thing, where the 3rdparty programs comes as a virtual machine image with a complete OS in it, with own kernel for maximal platform-independency.

Isn’t this kind what java is already? And also if there is another OS running in this VM, this would lead to less efficient executables as you’d have to partition resources like mem