Um, WW isn't even close to being cleaned out. People are already claiming GSP is ducking Hendricks. You can even find a lot of thread of people saying GSP is afraid of Hendricks. Maybe GSP should clean up WW before fighting outside his weight division.
The fact is, GSP is about ten pounds lighter than Anderson (which is why Anderson can't make 170), and given a fight between two dominant champs, the heavier guy almost always wins. Anyone who doesn't think Anderson is a heavy favorite against GSP is smoking something. So given that we already know the outcome of Anderson-GSP, the only reason for the fight is to make money - and GSP is still early enough in his career that he doesn't need to make super fights for cash yet.
The reason Anderson wants GSP is because he's near the end of his career, and wants the big payday. It does nothing for his legacy to beat GSP - he's the bigger fighter, he's expected to beat GSP ... and it would be a huge upset if GSP won (about as much of an upset as Ben Henderson beating GSP). However, the fight could earn Anderson a lot of money. GSP has plenty of time to earn that money, so the fight makes way less sense for him. Again, how many heavier fighters have gone for super fights when they were still in their early 30's?