Here's a simple way to think of the whole Anderson Silva Vs GSP situation.
Anderson Silva = #1
George St. Pierre = #2
If GSP ever wants to become #1, he has to take the fight. If he loses, he's still #2. If he wins, he becomes the greatest of all time. For GSP, there is no losing position for his legacy.
I'm probably the best at keeping threads on topic of all time.