You're right, there's a really good point here that I didn't acknowledge. The way I think of it is that the browser's security model, while not particularly sec

You're right, there's a really good point here that I didn't acknowledge. The way I think of it is that the browser's security model, while not particularly secure from an academic or theoretical standpoint, is extremely widely deployed, widely studied, and well-understood, and in that very pragmatic sense is very well-trusted. Hence, every little thing that affects the security surface area must be exquisitely scrutinized, necessarily resulting in slow advancement in any such features.