iOS apps can become pretty complicated endeavors with all the moving pieces. It's as complicated as writing any desktop app. In fact, many are currently in the situation where you need to move from an OpenGL implementation to a Metal implementation. I don't see that happening in the browser.

Though, I will say once you get that move completed, you get this: https://developer.apple.com/videos/play/wwdc2018/608/

Metal is nice, but Apple should also support OpenGL, not just WebGL.

That would be wonderful, now that I think of it. The only hard problem would be the simulator, which would probably have to run on bare metal, or as a virtual machine.