I'm trying to make the argument that macOS (Mac OS X) has a very long legacy as a desktop operating system, but my colleagues brought up the point that it's gone through so many iterations since 10.0 that it might as well be considered completely different.
My question is this: Is there any way to run even one GUI application on both Cheetah and High Sierra?
Dirty tricks can be used, but it has to be the same distributable (e.g. .app file). I don't really care if it's a fat binary, or if I have to install a different window server or anything like that. As long as I can somehow demonstrate to my colleagues that I have a GUI application that I can run on Cheetah that also runs just fine on High Sierra.
Best Answer
According to Wikipedia, Cheetah was shipped with AppleScript. So if you can have a Mac running Snow Leopard, according to comments of this question (and not the answer, it's strange), it seems that an AppleScript saved as app give a universal app (PowerPC G4 and above, or Intel). I haven't tested, but perhaps if you save this AppleScript:
as app on the Snow Leopard's AppleScript Editor, it will run under Cheetah and High Sierra.
This AppleScript app is very simple, and display "hello" in a window, with a "OK" button. So this is a very simple GUI application.