It's true that if all the code works well, is tested and all the features are supposed to stay the way they were when the code was written, then, any developer

It's true that if all the code works well, is tested and all the features are supposed to stay the way they were when the code was written, then, any developer can maintain any amount of code, there is just nothing to do. The problem arises when there is a change in what we want the code to do. Changing a feature that is implemented over three codebases in three different languages is definitely much more work than updating something that was written in SQLPage, for instance.