I don't think his retiring will hurt boxing.
Many people had grown tired of him as a human being.
As for being the best of all time I think you have to put that in perspective. From a lot of folks point of view in order to be considered "great" on has to have "great competition".
Back when Larry Holmes was dominating the heavy weight ranks as world champion no one spoke of him in the same terms of (Ali, Frasier, Forman, or Norton). What makes someone great is the level of the competition they had to contend with. Sugar Ray Leonard had Hearns, Duran, & Hagler.
However it's not Floyd's fought that he reigned in era that was devoid of marquee fighters. Ali is considered a legend. Floyd will never be seen as such.
Ali was bigger than the sport of boxing. He had to overcome being stripped of boxing license for 3 years during his prime. He protested the Vietnam War, dealt with racism, won the championship three times and fought the biggest and baddest guys of his time. Not only did he win fights he predicted the rounds he'd knock people out. Ali's last fight was December 11, 1981. That's almost 34 years ago.
Something tells me that 34 years from now Floyd Mayweather, Jr. won't have postage stamps with his face, movies made about him, songs written about him, nor beloved by millions of people around the globe. Ali changed boxing from a backroom gym sport to filling arenas that brought out world famous celebrities and multi million payments.