The reactivity layers are all pretty tied into the hearts of the frameworks. There's no advantage to any framework to expose such a thing to end users to leverage a competing implementation.

As for polymorphism, even the current class syntax largely operates in the same way as the original prototypal inheritance mechanism, with a few exceptions in constructor behavior to support subclassing certain built-in objects.

You can pretty easily create run-time traits- like functions with prototpyes, the class construct is an expression whose resulting value can be passed around, mutated, etc.

For example, you can write a function that takes a class as an argument, and returns a new class that extends it.