I've made a Windows GUI for a powerful command line open source application that was for Linux. It makes a few thousands dollars a month. It did take many years

I've made a Windows GUI for a powerful command line open source application that was for Linux. It makes a few thousands dollars a month. It did take many years of part time programming along with user feedback to get to that stage though. Desktop and Windows might seem like a dying market but that's what people use at work and those are the people who can pay for things.