In Opeth's case, I find any discussion even remotely related to "sell-out" a load of crap. My ears don't hear "sell-out". I hear Opeth, a band that has grown over the years, but has not lost touch with what makes them great. They continue to amaze me, and I find GR one of their best albums, and Ghost of Perdition my favorite song, period.
I would love to see Opeth spiral to the heights Metallica did, because I find them totally deserving based upon the music they have given me. They have struggled financially to bring to us loyal fans a style of music like no other. It's time they were repaid. I could care less if they have a large fanbase. Nor do I care if most of the fans don't truly appreciate what the band really is but instead are on a bandwagon. All I know is I'm thoroughly into Opeth, and they will most likely continue to be my favorite band for years to come. Led Zeppelin was my favorite band for over 30 years until Opeth came along. I don't see any band on the horizon taking over the top spot from Opeth, so for all I know, I'll go to my grave with Opeth being my favorite.