How much does it count? In some cases, it shouldn't count at all. Larry Holmes butchering a fading Muhammad Ali is one of these cases. Ali was so far gone that it's a travesty that they let this fight go take place, especially with the knowledge of a hole being in Ali's brain at the time.
Joe Louis versus Rocky Marciano is kind of a gray area in my book. Joe Louis was past his best. That's a fact, but he was still fundamentally sound and beating most of his opponents. He was outpointed by Ezzard Charles, but the fight was a close one and Louis closed Charles' eye. It says a lot that Louis gave such a great performance against someone so much younger. I personally think that Marciano would lick any version of Joe Louis because of his style. Louis hated pressure. He said it himself. Marciano beat a washed up Joe Louis, but Louis didn't embarrass himself and showed flashes of his former self at times. He still packed a wallop, too.
Speaking of Joe Louis, what about his string of victories over former champions? Max Baer, Jimmy Braddock, Primo Carnera, Max Schmeling, Jack Sharkey, etc. All of these men were long past their best. Some were 12 years older than Louis, but Louis knocking them out was always considered a fair win. Granted, Joe Louis would crush these men in their prime, too. But it's still something to think about.
How about Lennox Lewis versus Mike Tyson? Mike Tyson lacked the timing and accuracy of his youth but he was still a fast and explosive puncher. Mike was mentally over the hill by the time Lewis got to him. He had the physical tools to beat him but his heart was not in the sport anymore. I think David Tua was mentally exhausted when Lewis fought him, too. Tua was much younger than Lewis but was never the same fighter after his controversial loss to Ike Ibeabuchi. You can tell Tua packed it in after that fight because he neglected his skills.
Mike Tyson versus Larry Holmes should count, in my opinion. Tyson had the style to beat Larry regardless. You also have to take into account that Larry had a great second career after his loss to Tyson. He gave prime Holyfield a tough fight, made easy work of a prime Ray Mercer, gave a good performance against Oliver McCall and had an easy win over Butterbean at the age of 52. You could argue that Holmes was never over the hill.
Joe Louis beat a bunch of men much older but they were still active when he fought them.
Some people say that Archie Moore was past it when Rocky Marciano beat him but that isn't so. Archie went on to win the light heavyweight championship again. He was too small to hang in there with heavyweights. (He was knocked out by Floyd Patterson and Muhammad Ali too.)
I feel bad for Lennox Lewis because he beat Tyson and Holyfield when they were not at their best.
Funny you brought up Tyson's fight with Holmes. I think Larry would have done a lot better against Mike if he were younger and tuned up. When they fought, Larry was still good but nowhere near as good as he had been in previous years. And he took the fight on last minute's notice and didn't have time to get ready for Mike Tyson. I think prime Larry would get a decision over Tyson maybe, particularly since Larry had such a strong chin. But even though Larry was not prime when they fought, he wasn't a washed up has-been either. He did really well in later fights as you mentioned, performing well against Evander Holyfield and Oliver McCall and even winning clearly against Ray Mercer.
If Joe Louis had fought Rocky three years earler, he would have beaten him. The Joe Louis of 1947-48 was showing signs of age but still at an all-time great level. Jersey Joe Walcott made Joe look even older in those fights by boxing circles around him until he was knocked out. By 1951 Joe Louis was about done. Rocky's big punch was too much for an old man his age. I think Rocky is givin' too much credit for retiring Joe Louis but the fight was a big milestone in both of their careers.
^^I don't think Rocky is given too much credit for beating Joe Louis. Joe Louis was outgunned from the start but fought a competitive fight and even won quite a few rounds. (Although Rocky held back from rounds 1-6).
Joe Louis had previously knocked out Lee Savold and barely lost a close decision to Ezzard Charles. But with his timing, handspeed, and accuracy gone, it was only a matter of time before Louis would meet the guy to permanently finish him, and it turned out to be Marciano, who not only packed a heck of a punch but pressured Louis non-stop, something Louis always hated. If you ask me, Tyson and Marciano had the styles to beat Holmes and Louis regardless of when they fought. It's a styles thing.