What would it take to create an analog of SDL- some kind of lowest-common-denominator interface for mouse/keyboard io, audio output, and software-rendered graphics?
Something like SDL could be ported to run on top of Cosmopolitan but we'd need to port all its dependencies too. That would be a massive undertaking. If Cosmopolitan ends up being the next big thing and attracts a large community of contributors then something like that is bound to happen. Right now it's just a scrappy ambitious project being built for fun. So GUIs aren't something we're able to do quite yet. Although we've got TUIs working great! You can have the best most portable console / terminal apps in the world, today.
Think about it this way. How many years did it take Microsoft and Linux before they could offer a polished GUI? That should give you a rough estimate of how long it takes to develop these sorts of things from first-principles.