Cross platform development is EASY
It shouldn't be hard at all for find a developer who can make a GUI that runs on multiple platforms.
Java does it, .Net/Mono does it. There are no real differences, a button is an instance of the button class regardless of the underlying OS or graphical subsystem. This is precisely the main drive behind these languages. Sure there may be a difference in IDE but anyone who has used an IDE long enough will be able to adapt and anyone who hasnt will just see it as part of the same learning curve as before, assuming they do the development on the LInux system itself! They could just keep using their proffered IDE and simply test on a Linux system.
My god you could even use the most universal of cross platform of interfaces, HTML!
I have developed stuff for android. The IDE runs in Java, on any OS that runs Java. The VM that I test on runs on any platform that runs a supported Java, heck I usually just forgo the VM and push the app to my personal phone, using USB! With full debugging, live over USB. In Linux, just by plugging it in. I dont do it in windows because I cba to hunt down the driver I need.
Also, if anyone is using Visual Basic nowadays to do anything other than making a GUI for prototyping I'd be very worried.
And windows PC's can run virtual machines nowadays. Download a development environment turn-key VM, run it in a hypervisor. Develop on windows, compile, move to the Linux VM, run.
I've been doing this since the early 2000's there really is no excuse any more.
Once all the kiddies with their Linux running raspberry Pis grow up and expect to program on Linux because "everyone had a linux RPi" we will see the opposite argument I bet: "Nobody will develop a GUI for windows because everyone runs Linux and ts hard to find developers who can write a GUI for windows".
Everyone running Linux would be great of course. But that argument would still be stupid just as it is now. Cross platform development is EASY. The hard stuff was done by other very clever people a long time ago.
Hey, did you know? Minecraft runs on Linux, and windows. One executable. What is this sorcery?