Ross had a carrer and then some. Top 15 ATG in my book. Some tough, grueling fights and going 15 rounds with a prime Armstrong would probally persuade more than a couple fighters to hang em up. He retired at the right time and Marvin Hagler certainly retired at the right time. He couldn't persuade Leonard to sign for a rematch so what else would be a smart move for him?
Barny Ross did that 3 years before Armstrong repeated the feat with the lightweight, jr welterweight and welterweight titles. 1933 I think.