You can incrementally refactor existing code to make it more maintainable and easier to implement the things you are working on.

The issue with Netscape (per that article) was that it was a rewrite, not a refactor. I.e. replacing all the existing code with new code. That's why it took a long time to release a new version: the previous code hadn't changed and the new code wasn't functionally equivalent to the existing code, let alone have any new features.