Mayweather may have had the ability to become an ATG, but avoided too many prime fighters along the way and cherry-picked limited (Baldomir, Mitchell, etc) or faded (Oscar, Mosley, Hernandez, etc) opponents.

He avoided:

prime Cotto
prime Mosley
prime De La Hoya
Kostya Tszyu
Vernon Forrest
Paul Williams
Antonio Margarito
Manny Pacquiao

Floyd decided to "retire" when the welterweight division was at it's best in 2007-2009 with Cotto, Margarito, Williams, Mosley, etc.