Damn shame to hear that they're moving away from WoW raid boss guides (I've a feeling this will cost them quite a few views and subscribers), but given what they said about guild dramas in the OPs link to their weekly vid (I bet we've all been there re: guild drama) I can understand.
I usually just used them, icy veins and LFR to get a basic feel for an encounter, then just nutted out a strategy with my guild, so ultimately this isn't really a major loss for me. But I still liked their guides and its sad that the community as a whole is losing a resource.
According to the vid OP linked, they seem to wanna move into being some kind of general gaming channel, and seem to have had a big loss of interest in WoW due to guild dramas (And probably other issues). But....I subscribed to their channel and liked their vids as a raid guide channel. So for me, I've already unsubscribed and I have a feeling they may start to see a drop in subs and views until they build an audience that's looking for what they wanna do. I'm (currently) not among them, but still wish the Fatboss guys luck.