I just worry it's going to be abused, though. E.g. I've worked with more than one Ruby code base where someone did a kind_of? check and threw exceptions if it didn't get what it wanted even though the actual type required was anything that implemented a given method in a reasonable way for no good reason.
I hope people keep the type annotations sparse, and allow the tools to infer it unless they're prepared to link long and hard about the minimal restrictions that are reasonable.
I think your own example proves that your concern is moot. If people are going to do stupid stuff, they're going to do it with whatever tools are available to them. Your kind_of misbehavior already happens without type annotations, but now it can be clear to you beforehand what's going to happen.
I hope you're right. I just fear that making it external to the code will make it easier for people to ship overly restrictive type signatures without thinking. Though hopefully the tools like sorbet will make it easy to override, in which case it might well improve things (if I "only" need to override the type signatures instead of having to monkey patch or fork code)
I hope people keep the type annotations sparse, and allow the tools to infer it unless they're prepared to link long and hard about the minimal restrictions that are reasonable.