silva has palhares,falcao,santiago,stann,sonnen rematch,bisping and maybe lombard if he ever signs with the ufc. he has plenty.
gsp has diaz,story,ellnburger,macdonald,condit and rocha if he gets better stand up.
I understand what you're saying but a lot of people dont really think any of those guys have a shot at GSP and that would lead to lower PPV sales. Dont get me wrong Im one of the people that think Diaz would have a good chance and probably be his toughest fight but other than that the most likely outcome is he beats the others by a wide margin, everyone knows that and his fights lose a little bit of appeal.
With the exception of Sunnen, I dont see any challengers that I look at and say hay this guy "should" beat Silva.. except maybe and I do know its early but Jones I think would beat Silva.