This is just my opinion, but... I don't see GTK and Vala lasting very long. For one, GTK3 doesn't even have a windows version, there are some random people who

This is just my opinion, but... I don't see GTK and Vala lasting very long. For one, GTK3 doesn't even have a windows version, there are some random people who made one, but no community supported windows version. GTK3 is also less popular then QT, GTK3 only being more popular on Linux. While QT looks better and runs better on both windows and mac. I like Vala and GTK and I've contributed to elementary OS for a while now. But every day they get questions about why they use Vala. So... People don't generally like it or use it. But... really use whatever you want. If you like or need C, use GTK. If you like or use C++ use QT. If you use C and want a little more, try Vala. If you need Windows or really good mac. Use C++ and QT, or do the backend in C or C++ and platform specific frontend. Even with QT5, Mac users will know, because certain things still look off Another popular idea seems to be using HTML and JavaScript for your frontend with things like CEF.