Could Roy Jones upset Ray Robinson?

If fighters today are so much more advanced and superior to yesteryear's fighters, why can't today's fighters match the achievements of yesteryear? For example, Although the rules of today's boxing won't allow anyone to hold 3 titles in 3 different weight divisions like Henry Armstrong did, why couldn't Floyd for example successfully defend the welterweight title 19 times in 2 years like Armstrong also did? Why couldn't he fight and beat all the best welterweights in the world without ever losing at welterweight like Robinson did? Why has no heavyweight yet broken Rocky Marciano's record? Why has no heavweight surpassed Joe Louis in length of title reign or number of title defenses?

 

I believe it was either 1940s or 1950s when the National Boxing Association desided to really put in force the rule that the champion must defend his title at least once every six months.