BJ Penn is a megalomaniac, he thinks that he is the best no matter what ... regardless of failure.
Anderson Silva is a large enough man to fight at either weight, and 185 has historically been the second thinnest division next to HW.
GSP is a 'big' WW. Not the biggest, certainly. Both Alves and Fitch weighed more than him when they fought. He is also the youngest of the fighters, and is arguably just as talented.
While I agree, a fight with GSP and AS would be epic as far as reputation goes ... its a dumb fight. Would BJ fight AS? Id love to see it. It'd be like Chael vs. AS except instead of Chael getting takedowns it would be AS beating the fucking shit out of a smaller fighter ... devoid of the wrestling skills that someone like GSP has. GSP vs. AS? Very similar, except GSP might get a takedown or two.
We are here to drink beer. We are here to kill war. We are here to laugh at the odds and live our lives so well that Death will tremble to take us.