See Brian Goetz's talk on language stewardship (https://www.youtube.com/watch?v=2y5Pv4yN0b0). I'm sure nobody wants to add new and interesting features to Java more than the people who work on the language every damn day, but they've made a strong commitment to the community to not break existing code.

C# shows pretty compellingly that adding features does not have to break existing code. For example, you can use every keyword added after 1.0 as identifier, too.

But it can make things more complex too. e.g. I recently learned that the main reason that asynchronous C# methods have to be declared "async" is so that the compiler knows that in the method body, "await" is a keyword. Without this backward compatibility requirement, it could be inferred from the return type + the presence of 1 or more "await" keywords.