Note that Java makes breaking changes all the time, which is why it publishes a compatibility guide with each major release. These are usually judged to be minor breakages, but if you have a codebase on the order of millions of lines, there's a very good chance that at least one thing will break and require a little bit of work to upgrade. And Java's not unique here, every stable language makes changes all the time that have the potential to break some user in some edge case.