No. I reckon Foreman being able to hold his own in the ring later in life, shows the gulf in quality between him and his opponents when he was in his prime, and the more modern boxers.
If the late Teofilo Stevenson had turned pro he would have wiped the floor with most of the heavyweights in that era, including Ali and Foreman, and Smoking Joe..
I'm certain that I'm not sure. I'm even less certain having looked at Mercer's Wiki page. Yours looks like a good call to me. https://en.wikipedia.org/wiki/Ray_Mercer Spoiler 2nd paragraph. https://en.wikipedia.org/wiki/Anthony_Joshua He is also the second boxer, after Joe Frazier, to win a world heavyweight title whilst still reigning as Olympic champion at the top weight.
If you're looking at unfulfilled potential then Butterbean is your man. He could have been the greatest ever. If you don't know who he is then you know **** all about boxing. FACT.