Objective-C is a good example of successfully migrating the language forward. Breaking changes were introduced slowly. Library APIs are marked as deprecated sev

Objective-C is a good example of successfully migrating the language forward. Breaking changes were introduced slowly. Library APIs are marked as deprecated several releases prior to removal. The compiler warns you of their use. Automatic reference counting can be turned off for legacy files, but a tool is provided that will help you convert a class to use ARC. Code written in ObjC a couple of years ago will likely not run today without modifications. But overall this has been a net positive for the language. It's actually become really pleasant to use. And is in part why Apple's platform has thrived. I think optimizing for the past is the wrong thing to do. If your language is successful, then most of the code is yet to be written. So if your choice is between making future code easy to write, or not breaking legacy code, then you should err on breaking legacy code and provide deprecation/migration tools.