I always liked Rich Hickey's point, that you should program on the inside the way you program on the outside. Over the wire you don't rely on types and make sure the entire internet is in type check harmony, it's on you to verify what you get, and that was what Alan Kay thought objects should do.
That's why I always find these complaints a bit puzzling. Yes in a dynamic language like Ruby, Python, Clojure, Smalltalk you can't impose global meaning, but you're not supposed to. If you have to edit countless of existing code just because some sender changed that's an indication you've ignored the principle of letting the recipient interpret the message. It shouldn't matter what someone else puts in a map, only what you take out of it, same way you don't care if the contents of the post truck change as long as your package is in it.
that's not a big deal, when we exchange generic information across networks we parse information all the time, in most use cases that's not an expensive operation. The gain is that this results in proper encapsulation, because the flipside of imposing meaning globally is that your entire codebase is one entangled ball, and as you scale a complex system, that tends to cost you more and more.
In the case of the OP where a program "breaks" and has to be recompiled every time some signature propagates through the entire system that is significant cost. Again if you think of a large scale computer network as an analog to a program, what costs more, parsing an input or rebooting and editing the entire system every time we add a field somewhere to a data structure, most consumers of that data don't care about?
this is how we got micro-services, which are nothing else but ways to introduce late binding and dynamism into static environments.
