Your premises are too flawed for me to respond to this question.
Boxing has not evolved. Not one bit. If anything, it has devolved. Heavyweights are bigger... and slower. Training methods are the same as they've always been. Fighters are gassed out now after 12 rounds; forget 15. Champions only fight twice a year, and nobody can stay at any given weight class for more than a few years.
Put Joe Louis at 200 pounds in the ring with the Klitschkos and he knocks them out on the same night.
Poet has a great article on the falsity of the "modern myth". You should give it a read.