Floyd doesn't give a flying fuck about beating Marciano's record. there's nothing in it for him. All he's interested in now is being classed as "the best ever." Marciano, for all what he did isn't ever classed as the best ever, so if he beat Marciano's record, anti Floyd people would say "so what?"
After 30 years in the game, Floyd has justifiably had enough. He's only interested in fights that cement his legacy. Fighting Cotto and become World Champ in another division might do it in some people's eyes, beating GGG might do it in some people's eyes, and that's about it.