If the JSON file had a version number for the spec, then could could support future versions without breaking it, ever. Just define that all future versions will include the previous versions. E.g., specify that if a file is meant to be compliant with the next JSON version, then it needs to start with

#JSON1

And the next one with #JSON2. And if a JSON2 parser finds a JSON1 file, it should interpret it as JSON1. If a JSON1 parser finds a JSON2 file, it should try its best to parse it anyway, because your changes are not going to arbitrarily break stuff, right?

This is a breaking change, too, but it may indeed be the last one...