Yes, method missing is a far more limited alternative to proxies. But its not a use case for proxies, they just have intersecting sets of use cases. It doesn't make sense to "just" implement method_missing, you would do that so that you can implement other things on top of method_missing.

@observable does not need proxies. Only implementing operator[] does. Which also could've been done with a far more limited feature, namely adding a symbol for the [] method.

Proxies are just silly.