I'm not sure if these are the best of examples. Norwegian and Swedish are by and large mutually intelligible, probably on a level comparable to Australian and U

I'm not sure if these are the best of examples. Norwegian and Swedish are by and large mutually intelligible, probably on a level comparable to Australian and U.S. English. Norwegians essentially are speaking Swedish, without trying. Finland has had a large (currently ~5%) Swedish-speaking minority for a long time. While a large amount (~50%) of Finnish-mother-tongue Finns can speak Swedish, this is expected given the Swedish minority, the previously large and prestigious role of Swedish in Finland, the relative populations (~5M vs ~9M for Sweden), and the relative linguistic isolation of Finnish. I wouldn't claim the proximity theory holds no water - it's just not the entire story.