This is probably one of the few times I'll agree with LyotoLeigon, and you never really addressed his point.
Silva has fought at 205 and done just fine, GSP has never fought above 170, and is quite a bit smaller than Silva.
If Anderson has nothing left to prove then why does he want to fight GSP so bad? A lot of "p4p" lists actually have GSP at #3, behind Silva and JBJ. JBJ would be the more challenging fight, and it infuriates me to see the supposed #1 fighter duck away from his biggest challenge yet.
I also really don't get what you mean that Silva has nothing left to prove at his age. Silva/Dana are the ones claiming this is the #1/#2 p4p fighter, or the "two best" fighters, yet again, most reputable lists have GSP at #3. Silva can prove he is the absolute best with a win over JBJ. I really don't think he proves that with a win against GSP.
I somewhat agree with you when you say Silva hasn't faced top quality opponents, but he has a top quality opponent available, at a weight class he's tested the waters in, and would rather take on someone lower ranked, and smaller.
But I agree with you 100%. Dana is a promoter, and says a lot of things and then changes his opinion a day later (just go back and look for a video on his stance on women in the UFC).
There is a reason Dana said this one night before the GSP / Condit fight (before GSP/Silva have formally agreed to anything). I'm thinking to probably light a fire in Condit (piss him off a little - make him feel he's being overlooked). There's also a reason why Dana is saying Silva will be watching cageside. You better believe Dana wants Silva to enter the cage and challenge GSP in front of his hometown fans. Dana knows there is a better chance of a GSP/Silva superfight, than a Silva/JBJ superfight, so he's going to do whatever it takes to sell 60,000 tickets at a stadium show. He doesn't care if its the "best" superfight or not - he knows either fight would sell out a stadium.
You're line of thinking is based too much on street bravado and not nearly enough on what makes financial sense. Going down in weight to get a bigger purse has been happening for decades in boxing (hell, a good number of pacquiao's opponents have gone down in weight just for the paycheck). GSP sells the most PPVs and gets about 5 million a fight and imo silva would be crazy not to bank on that fight first.