GSP and Silva have reached levels of success that people dream about. They got to the very top and were the fighters everyone loved and looked up to. But fame and privilege gives people less of a drive at times, and pride comes before the fall.
I feel like Silva was at an all-time low publicly before he fought Chael. He just had an absolutely embarassing fight against Maia and his star fell about as much as anyone could imagine in just one fight.
GSP is in the same situation right now. His long absence and the HUGE amount of criticism he's been getting for safe fighting has made him the lowest he's ever been in terms of glory and fan reaction. Diaz is just capitalizing on this the same way that Chael capitalized on Silva's spot of weakness.
It's funny cuz GSP/SILVA still are the top 2 guys, but it just goes to show you that there's always going to be new people that are hungry and more driven coming for that spot, and you can't rest on your past accomplishments