With all due respect to Loma, the key words in your sentence are "would have". The fact is that he didn't. There were many other boxers that maybe could have matched their accomplishments if they stayed in the amateurs for their whole careers (like Rigo for example) but they didn't. Like I said, Lomachenko is still one of the best amateurs ever, but the fact is that he is far from being considered the greatest and to be honest the mentioned boxers were far more recognized and accomplished than him.
Lots of people could have outshined them if they stayed amateuir their whole fighting career. You think Andre Ward for example couldnt have gone on and won multiple olympic medals? De La Hoya?