In my opinion it's not the fact of when or if Silva/GSP will lose before they retire. It is more the fact that it has been such a dominant stretch by both fighters, one i can see Jon Jones emulating also, people like to build guys up and put them on a pedestal, trouble is when they are on that pedestal people in general take more satisfaction in the fall. What ends up happening is you have people fighting these guys for the title that don't really pose the threat that everyone hypes them out to have.
Saying that I think they will both lose at some point. How Silva is even fighting at the level he is at his age still boggles the mind. One thing you can be certain of is when or if Silva loses he will get the 'but a prime Silva would have won' As for GSP I'm sure some excuse will be made, I just hope GSP has at least one more war in his career or at least a performance that his die hard fans can use to prove he actually still has the heart of a warrior instead of this control and hump strategy his camp have him employing...