If a band is good, it's good, so if I've followed them from their indie days there'd be no reason to stop. However, I would probably stop going to concerts if they were playing huge venues like the Giant Center or whatever, because I hate huge crowds like that. So it would be sad.
I'm sure there'd be no shortage of small, local bands to replace it, and I would still consider myself a fan, unless they turned into douchebags like those assholes in Metallica.
Conversely, mainstream appeal is often a turn-off when getting into new bands. Music is usually not very creative at that level, with some exceptions.