So basically your argument is that an accent or dialect is only valid if it can be understood by people from outside that speech community? German and English both have the same linguistic origins, but they are not mutually intelligible. Does that make either or both invalid ways of speaking? Do you realise that phonological changes are a perfectly natural part of linguistic evolution and, given enough time, speakers of dialect X and dialect Y can stop being able to understand each other? Yes, you can code-switch to make yourself more understandable to speakers of another dialect, but that’s generally what you do when the differences between the two dialects are big enough that you feel the need to change the entire manner in which you speak.