You don't HAVE to do Champ vs Champ to be a superfight. I would consider a fight like Jon Jones vs Junior Dos Santos to be a superfight, for example. But in most cases, a superfight could be champ vs champ: GSP vs Penn II, Hendo vs Wand II, Hendo vs. Anderson, Hendo vs. Rampage... Maybe not all of those, but most would be superfights. You've got huge fights like Fedor vs. Hendo or GSP vs. Shields as well. There's a lot of great fights that can be made that would be extraordinary or above the normal marquee bout. On paper, I don't think Anderson vs. Diaz meets that standard. You've got two aging fighters who have recent definitive losses, and there are question marks surrounding both. Anderson is around 40 years old, coming off a serious injury, and Nick Diaz admittedly doesn't even want to fight anymore and lacks passion for the fight game. Doesn't sound like a superfight to me, though I'm sure it'll be interesting for however long it lasts (not very, most likely).
Specialization helps different disciplines advance. MMA keeps them practical.