I think the key issue is being overlooked. It's not about the position of PG, it's about 6'nothin nikkas aka the majority of them. If you best player is only 6'0 that puts limitations on what you are able to do offensively and defensively. Then add to that you have more and more taller players who are able to give you what traditional PGs are known to provide. I'm really not sure what the argument to the contrary is.
Yes we can talk about the top PGs not having championship casts but the rebuttal to that is what exactly does it takes to put a championship cast around a PG? If you replace Lebron with CP3 or Westbrook or Rose
the Heat win it all the last couple years? How about in Dirk's spot in 2011? Or maybe Kobe's in 09, 10? Pierce in 2008? Duncan in 07? Wade in 06? And so on. Conversely who beats these Clippers with Bron, Kobe, or prime Dirk, Duncan in CP3's spot? Who beats the Thunder with any of those guys in Westbrook's place? With bigger guys you can move them around and have them play different roles if their skills happen to overlap(see Lebron/Wade). Smaller PGs aren't as versatile. Clips had to trade Bledsoe (who is beasting) because you can't have two small guards.