The evidence for it isn't just anecdotal though, Spotify crunched the numbers and found metal to be the most 'loyal' genre:
'To find out, we first identified the “core” artists that, according to The Echo Nest (a part of Spotify), are most central to each genre, starting with the big ones, on a global level. Then we did the same thing with local genres in various countries around the world. To create a measure of genre loyalty, we divided the number of streams each core artist had by their number of listeners. All of the charts are normalized against the genre with the loyalest fans.'(Picture: Spotify)
After Metal and Pop, the remaining genres cluster into two groups. Folk, Country, Rock and Hip Hop all have fairly loyal followings, while Jazz, EDM, Classical, and Blues seem to be genres people are more likely to 'dip into' every now and again.
The data could of course also be interpreted as negative for metal, suggesting fans restrict themselves to the genre and are less likely to broaden their horizons with different sounds.
Looking at genre loyalty by country, Spotify found Britain to be dedicated to 'British Alternative Rock' (presumably Oasis, Primal Scream etc), while the US was far and away most loyal to Mexican music.