I always look at music in my own little shell. I don't particularly care about any fan base, and how that base changes over time. Of course more people, no matter how small, will hear Opeth for the first time, thus growing that base.
So what? Other than album availability. (actually, I ordered their entire library all at once @ CDNow.com just a few months ago)
Hey, I found Opeth somehow, and they are now my favorite metal band. My taste for them has not waivered in the least. I know I hear about "mainstream" acts first, because they are the ones who get national headlines, and make all the money. I have never radiated to bands like that.
I really agree with Despot in that Opeth becoming big would mean they were making money, so they could tour more frequently, and turn many more people on to them (as long as they didn't change anything because a promoter decided an "image change" would even make more $$).
I think the ultimate horror would be to see Opeth get big by following in the professional footsteps of the likes of Brittany Spears, N'Sync, Backstreet Boys, and even to an extent Aerosmith and Metallica (more recently). Even knowing they were making all that $$, can you imagine Opeth concerts rivaling the followings of concerts like Janet Jackson, Madonna, N'Sync, and others like that? (good? bad? not sure)
Maybe us Opethians are jealous in some way, and only want to share Opeth with select individuals whom we believe deserve Opeth, because they can understand their complexity and originality. Let's hide Opeth in some remote location, provide them with a state-of-the-art studio, have them make CD's for us, and come play in our basements.