It happens in every sport.
Joe Montana is the greatest QB
Gordie Howe or Wayne Gretzky is the best hockey player.
Michael Jordan...
and so on and so on.
The reality is athletes now are faster, stronger, and have more knowledge. So if someone every invented a time machine to put boxers in their prime against each other, I would usually put my money on the current athlete. I think what people should be arguing is that who was the most ahead of their time.