"So it's perfectly possible to target another Platform or do this as some kind of WebApp."

Yes, it is possible to rewrite software. But currently most of that software was written and licenced for windows.

Just choosing another plattform might, or might not work. And if it doesn't, many people will be angry for not getting tax refunds back, or getting a new passport, or being able to register a new car etc.

Bugs are real. And there is a saying, never change a running system.

So yes, I do agree that the system is not running so well being dependant on Trump and change is required, but this is not just some webapp for fun that needs replacement. We are talking about critical government services, with lots of custom made software, that was often exclusivly written for windows.