While I get some of the context you're trying to bring to light for Greb, on the flipside, you aren't doing the same for Tunney. As you yourself have said, Tunney no doubt continued to improve himself with each subsequent fight with Greb. So listing the first two fights, would by proxy mean, Tunney wasn't prime for those fights. He wasn't prime in the third but still won. The 4th again, he hadn't fully grown into the finished package yet, but some still felt he won. By the 5th right, he was almost the finished product and closest to his prime and dominated Greb. So Greb may not have been for any of them, Tunney was only prime for one of them, maybe two if we're generous, so I'm not seeing how all this adds up to a Greb for sure win in his prime. It's possible, considering how good Greb was, but I'm not seeing it as for sure based on their fights.
And so we get to Prime Tunney who faces a 5 year past it, inactive JD ..not impressed !! … Prime JD knocks him out easy .. If you are going to say that Tunney wasn't prime then Greb being shopworn is an equal or greater argument, imo ..especially considering how many more fights he had then Tunney … Although I do agree with you, that Prime for Prime and clean, it is one helluva a fight
I'm never going to say a prime JD would've knocked Tunney out easily, when he won one round out of 20 total. That is total and complete domination where Tunney clearly had the style to beat Dempsey at any point in his career. Let's be honest here, yes Dempsey was living high on the hog and complacent the first time he met Gene. However in the rematch, he was clearly motivated to regain his title and trained better, and still only won a single round of that fight. A good round mind you, but still only one round. I believe you'd need more than "he wasn't prime" to make up for that kind of disparity in rounds. Okay, but you're forgetting a few factors that played into Greb's early success. Greb was special because of his talent and intangibles, but also because of his style. It took some getting used to because he didn't fight like anybody else. It's very difficult to beat somebody who has a swarming aggressive style you aren't accustomed to, who also happens to have almost unnatural durability and stamina. That style would surely take some more getting used to than Tunney's more traditional straightforward style that Greb was used to seeing. Further, you're calling it shopworm, which in another argument it could be called experience, valuable experience. It's not like it's unusual for a "shopworm" great to beat an slightly green great. It's something we've seen throughout history occur. I happen to think this is a good fight prime for prime, and I also don't think they ever fought when both were at their primes, so the conclusions become difficult to draw.
To be fair and balanced, Greb's official win over Tunney happened when Tunney was a bit pre-prime. When Tunney filled out, he was too much for Greb. Greb might have been fearless, but he declined another fight with Tunney, knowing full well that Tunney became the better man. Sure, Tunney was bigger by then just saying... Tunney arc continued to grow as his career progressed, and might have retired before he hit his absolute peak. At their best, I'm picking Tunney.