This is unrealistic example and has error handling completely backwards. The exception handler in main() only knows how to handle specific types of errors -- it doesn't know or care where they are thrown, that's not it's problem. Lets say, for example, it knows it can handle network errors by retrying the operation.

If b() changes to throw a different type of exception that main() doesn't know to handle them main will break. And breaking is entirely the correct behaviour.

Maybe b() is an interface and the actual implementation might not even have existed when main() was written. Maybe yesterday it was implemented with a file, tomorrow with a HTTP call, and maybe next week something else. But tomorrow, at least, it can retry the operation when it fails.