Tunney was noted for his incredible conditioning, as well as his adaptability and ring smarts, so going 20 rounds would not be a big problem for him in my opinion. I see him winning a decision over Jeffries, although this is largely based on what I've read of Jeffries, not what I've seen considering there's not much film of him. He was not an all-out pressure fighter, but a more calculative fighter who wore down his opposition over the distance.
Dempsey looked "shot" (or at least seriously over-the-hill) against Jack Sharkey too, and the win was far from convincing.
My initial reaction was that Tunney would be far too skilled for Jeffries, and that regardless of the distance and the date, Tunney wins easily either by points or late TKO over an exhausted Jeffries. Then I remembered something.....by 1903, if you're a scientific, technically accomplished fighter fighting Jeffries, you had BETTER be strong enough to cope in a clinch. And this is where I have my doubts. Jack Johnson couldn't keep Jeffries off him, but he was strong enough to cope in the inside exchanges and battered Jeffries with hard shots at range with his superior speed. Tunney (I think) has a longer reach than Johnson, and so I can easily imagine him using his jab to befuddle Jeffries for long periods, racking up the points. He hit hard enough to hurt Jeffries, certainly, and there's every chance he might stop Jeffries in a similar fashion to Johnson. However, I personally see Jeffries producing a gargantuan effort, taking a horrific beating to get his work in, as he often did, smothering Tunney and tiring him at a quicker rate then Jeffries' endurance is depleting. I see the fight ending within around 18/19 rounds in Jeffries favour. In 1900, I see the bout going in Tunney's favour, although he'd be in trouble by the end of it.
Again, I can only say "shot" means something different to you than to me. To me, "shot" means finished. Shot fighters don't beat the #3 HW in the world, convincingly or otherwise.
Maybe if you hit them in the balls enough times. Anyway, if he wasn't "shot", Dempsey was severely past his best, and I think he was very lucky not to get an "L" against Sharkey. I dont put much stock in that win at all. If anything it heavily confirmed that Dempsey was over-the-hill.
I think that his being past his best is inarguably, but the degree is woefully exaggerated in this thread. I'm watching the fight now, I can see Dempsey winging combo's, slipping jabs and coming back from hard punches directly. Compare this to something like Tyson versus Lewis where Tyson was basically target practice for Lewis after about 140 seconds, and even here "shot" is likely to strong a word for Mike.
I do not think Dempsey performed that horribly against Tunney or Sharkey. Physically he did not seem to be worn out, he still had plenty of power, speed and toughness. If he lacked anything it was the fire of old, surely the result of his gained wealth and fame. Technically Dempsey was never going to outbox the likes of Tunney and Sharkey, he had to rough them up and KO them.
Dempsey was an inactive fighter past his prime attempting to come back off one too many layoffs, going up against good active fighters in their primes. He failed. I think "****" is the word you're looking for.
I think that's the safest way to say it, and it's certainly better than labelling him shot. Physically, he looks pretty good to me.