Vue reactivity isn't compatible with Svelte, nor Angular.
As a counter example to your question, what if we all had competing implementations of the object primitive. Libraries would barely work with one another and would need an interop layer between them (just as reactivity layers do today!)
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.
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.
I struggle to see what more one could want. Do you have any specific features you think would help a massive ecosystem of packages be able to work together, when for example different packages have different Signal implementations?