Can there ever be a "living" standard to replace posix ...? Something that captures the good things about posix but allows all the related ecosystems to move everyone forward toward a better base specification (together rather than just increasing the number of bsd vs gnu special cases that with current plans will just have to be lived with forever)

Browser standards managed to do this in a lot of ways despite far more complex standards, more complex variations in behavior, and much more rapid continue evolution ...

No, and that's a good thing. Just look at how quickly features are added to the living browser standards. Instead, we get a new POSIX version every couple of years that includes the special cases that actually work the same on BSDs and Linux after they're already implemented.