Do you have any data to support your assertion that newer bands generally grow their fanbase at faster rates than older bands? It seems sort of intuitive, but then history is littered with ideas that seemed sort of intuitive, but upon investigation ended up being flat wrong. It would be *more* believable if Novembers Doom had actually been exposed to their entire market of potential fans and was near their saturation point, but given that they currently have a mere 12% of Opeth's last.fm listeners, it seems there is still plenty of headroom there. I bet if Novembers Doom had opened for an Opeth tour, their growth over this period would have been much larger than it was (especially on the current Opeth tour, where fans might be willing to jump on *anything* that's heavy with growls!
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Perhaps a refined version of your statement, which then also exactly supports my assertions in this thread, is that *headlining* bands grow their fanbase at a slower rate than opening bands. And since there is a strong correlation between a band's age and its headlining status, then the tendency would be for older bands to accumulate fans more slowly. But not simply because they're older, but because they've stopped advertising themselves as strongly to new potential fans. Put even a 30-year-old band on a Metallica tour, and their fanbase will still surely jump.
Finally, one of the best takeaways from the data I posted doesn't even involve the three older bands. If we just look at the top three, bands all in very similar situations in terms of age, Blackguard had nearly double the growth of Mutiny Within and Powerglove (54% vs. 36% and 35%), which is a significant difference. I wonder why? Well, surprise surprise, they also played twice the number of shows over the past 2 years (273 for Blackguard, 135 and 142 for Mutiny Within and Powerglove, respectively).
Now, correlation is not equal to causation, so it's certainly possible that touring has nothing to do with it, in which case I guess the only explanation is that Blackguard is just flat-out awesome!
Neil