The thing is Holmes was not cleaning out his era like Louis, Tyson or Marciano did. Holmes didnt relly imo put a stamp on I was KING of the 80's, because of this, we have arguements over Holmes or Tyson as 80's heavyweights, with the favor of couse going over to Tyson for cleaning out the era. Marciano is special because he is, Even with a lost, he did win the title, rein with a iron fist, was the best of the 50's, and pretty much clean the era out with no arguing over who would beat prime Marciano in the 50's about. You will see no Wills for 7 years like Dempsey or color line arugment in regards to Rocky's ranking. Even with a lost, he should not be drop. He was to the 50's what Tyson was to the 80 s, Ali to the 60's and 70's, Louis for the 30's and 40's and of couse Dempsey for the 20's. He would still be a icon.
It was better than most. Sure its not Ali. But its compares to Louis, Tyson, Lewis and most evey one about.
Even with a loss on his record, he would still be one of a handfull of heavyweight champions who dominated a definable era, and that would have to be reflected in his ranking. Beyond that it would depend who he lost to and how. A loss significantly outside his prime would have less impact that one in his prime. The reality is that nobody did beat him and anybody available to do it near his prime would have been of a calibre that would have forced us to downgrade our estimate of his abilities. If he had chose to fight on untill he lost I suspect that age would be what ultimately defeated him.
If he was 49-1 and still beat the big names that he did beat it would not make any difrence. marcinao's resume is (and should be) measured only on his record in his key fights at contender/title level. Marciano knocked out all the linear champions active during his career and he always won emphatically at that level even if he retired before an inevidble fade. on the way up rocky like any good fighter was steered by great matchmaking. he was still pushed close on a number of ocasions on the way up during the grooming process but always either got lucky or got the benefit of the doubt. I would have no problem with him or any other great fighter dropping decisons in learning fights early on. it did not harm floyd patterson. I have always said joe frazier would be rated as high if he retired after TFOTC. frazier carried on and faded fast, maybe as fast as marciano may have had he continued. The problem was frazier missed out on facing former champs like floyd and liston who were active. what makes marcianos career perfect is not that he was 49-0, it was that he knocked out all the biggest names in the HW division from 1947-52 then ran out of chalengers. there wasnt any bigger names than louis, wallcot and charles from 47-52. joe louis did a simular thing knocking out the entire fleet of 1930's champions - only he slipped up with schmeling that one time.
If the loss came when Rocky was coming up,it's like a pitcher is being considered for Cooperstown, and someone wants to bring up a bad month he had when he was in the minors.