We've seen it with Liddell, Hughes, Royce, Fedor, Wanderlei, and so many others. You may seem unbeatable in MMA for a period of time, but age eventually catches up to everyone in the game.
In the new, ultra-competitive MMA landscape, extremely few people stay at the top of their division 35+
Anderson Silva may be a better fighter than Weidman and of course has the overall better career at this point, but all the UFC champs are basically 20 - 30 in today's UFC so I don't see how much longer Silva could keep on top. I think the thing that kept Silva apart from the rest is that he had a mystique about him that no other fighter was able to carry in the same way...when people just look at him objectively and evaluate his strengths and weaknesses, he may be just as beatable as any other champ. Between the Chonan submission, the first Chael fight, and the Weidman KO there's enough tape to see his holes and exploit them