I agree with some of the things the OP is saying. Top Level Boxers are world class athletes, they are the top 1% of people when it comes to athletic ability, GSP falls into that class as does Randy Moss and Kobe Bryant.
Kenflo isn't the best example because he was training in bjj almost as long as he was playing Soccer and I would also say that Kenflo is a phenominal athlete, playing Soccer at a collegiate level, earning a black belt in BJJ, learning Muy Thai and excelling in MMA IMO is the defanition of a great athlete. Is he "World Class"? No, but he's top 5%.
Rich Franklin is a better example, the guy was a School teacher who probably took some Karate classes as a kid, learned striking, grappling and subs in a short time and became MW champ. Evan Tanner is just a tough dude who watched some grappling instructional videos and took some kickboxing classes and made it to the UFC.
GSP is for sure the MMA fighter of tomorrow, in a few years you will have to be really good at all aspects of the sport as well as be in phenominal shape to be fighting at the top level in MMA. This will happen gradually as the pool of people who train increases and the sport becomes more lucritive.
That bieng said, If Floyd Mayweather, trained soley for MMA for the next 3-4 years could he be a force? I would say that he definetly could.
Good post, I understand where you are coming from