Sound decision by Joe. He's not getting any younger and if he goes on eventually he will come up against someone younger, stronger, fitter and faster who will give him a real beating. There are too many old boxers around (and many who are no longer still around) who have gone on taking 'one more fight' and paid dearly for it.
Oh, and by the way, as for only fighting bums, you should check out the credentials of some of Marciano's opponents too!
With most boxing careers its all about timing...Marciano was fortunate that the 50's was short of top class heavyweights. He missed Joe Louis in his prime and retired well before Liston and Ali appeared on the scene.In title fights he defeated former champions Jersey Joe Walcott and Ezzard Charles ( good fighters but not top class ) and also knocked over Don Cockell ( British punchbag ) and Archie Moore ( Light-Heavyweight )
His career has parallels with Calzaghe in that they both fought the bigger names when they were past their prime, either through good luck or shrewd management.
The record books will show them both with unblemished records but the debate will rage on as to how good they both were, as their individual records come under scrutiny.