I disagree about boxers being better athletes. Anyone can learn to box, and be very successful, but obviously the TOP guys will all be very athletic. Look at the UFC champs for example: Sean Sherk, Georges St Pierre, Anderson Silva, Rampage Jackson, Randy Couture. You think boxers are more athletic than those people?
You make some interesting points, but I think it's important to look at how long both sports have been around. As you mentioned, MMA is still considerably young, whereas boxing is a sport that has essentially existed since the 12th, in one form or another.
However, if you look at the early boxers following commercialization of the sport, they honestly don't stack up much differently to the Tank Abbott's of the early UFC. Compare Floyd Mayweather to a boxer from 50 years ago, and the difference is undeniable. Technically and athletically, Mayweather (or any boxer of this generation for that matter) is well beyond what pre-existed him.
I believe the growth and evolution of boxers however, is much slower than that of mixed martial artists. Look at early cagefighters such as Keith Hackney compared to the Matt Hughes/GSP's of the division. That drastic evolutiuon of one-dimensional martial artist/brawler to athletic mixed martial artist came in a matter of 10 years of less.
I'm not an expert in the field of boxing, but I do consider myself fairly knowledgeable about MMA, and I just have to say that MMA simply takes just as much athletic ability (if not more) than boxing does because there's simply more to consider.
You mentioned how boxers need quick reflexes, which is true, but reflexes and athleticism are aspects in all aspects of MMA, not just the striking aspect, unlike boxing.
Boxing is a part of mixed martial arts, so the transition to MMA isn't impossible. It's just very overwhelming considering how much there is to MMA. Boxing is one-dimensional, and I'm not saying that in an insulting manner, I'm simply stating there is far less to athletically/technically be prepared for than in MMA.
Kenflo may have been a soccer player, but I doubt Mayweather would be athletic/dexterous as Kenflo off his back. Athleticism helps in MMA, it helps in all sports, but its not imperative in order to be successful. Hell, look at Melvin Guillard. All the athletic ability in the world, can he apply it to MMA beyond striking and wrestling? Not really, he's been getting tooled by guys that are technically less athletic than he is.
Hell, Maradonna wasn't that athletic and he was still a hell of a soccer player.