If Georges is going to take a fight against Anderson, it's basically all risk and no reward at a catchweight. If he wins, sure he can say he beat the GOAT, but he gets no belt and people after the fact can second guess Anderson's weight cut. If he loses, he heads back to WW with a little more damage and gets little besides a big payday, but GSP is clearly about winning much more than money.
Fighting at a catchweight is a waste. I think the reason GSP was equivocating a bit last night was because he was so happy to get his belt back and has to think long and hard about giving it up to move to MW permanently.
If he left the WW division now, there are a number of challengers who could cobble together an impressive tournament to fill the void. If Anderson left MW, even though there are challengers, most people agree that they wouldn't fill Anderson's shoes in any satisfying way. Weidman, Bisping, Boetsch, Belcher; maybe Sonnen would return. Rockhold is out there. These are all talented guys and may even be able to give Anderson a little trouble, but no one is realistically thinking they could fill Anderson's shoes right now.
At WW, a tournament of Hendricks, Diaz, Condit, Rory/Penn winner and Marquardt would be very compelling and the eventual winner would definitely have the credentials to at least be considered a viable replacement now that GSP has moved up.
GSP needs to either stay at WW and face all these challengers (except Rory who he won't fight) or go to MW for good. Catchweight superfight is a losing proposition for his career and for the sport other unless they both immediately retired afterward.