That's an interesting take on it. I wonder if there's much history of groups starting out with common languages, then groups separating and the languages evolving separately, either intentionally to demonstrate group membership, or just due to lack of communication between groups, and continuing to differentiate until they're effectively different languages.

I only speak English, but I've heard several different accents on it so strong that they're hard to understand. I wonder if the speakers of these accents find it impossible to understand other strong accents.