I'll admit I don't use JavaScript very often, but surely the state of polymorphism could be improved? For example, C++ recently added concepts, and most (modern) languages have some way to describe interfaces.

As to your counterexample, I agree with current JavaScript that would be a problem, but with good language support it would certainly be possible. For example, Rust (and C++?) have competing implementations of the global allocator, and must users will never notice.

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.