I think Dempsey would have struggled in a lot of the fights especially later on. Much the same as we all more or less agreed that Marciano trying to duplicate Lewis' career would have resulted in abnormal wear and tear on the much smaller Marciano ... I think the same sort of thing would have happened in Dempsey's case. I don't see Dempsey knocking out Purrity, and I think a cutie with a bit of size like Byrd may have given him hassles. Guys like Povetkin that Wlad was able to use his bulk to dominate would very likely have been a gruesome slugfest for Dempsey. I think that Klitschko has aged so well because his style enables him to win while taking minimal damage. Dempsey's skillset would take him straight into a brawl with much bigger guys.
Wlad was on top for so long. Dempsey has quite a few losses if he faces the same guys. I'll say 4-5 losses by age 35 ( Dempsey's style would not age well ) , and several more past that age.
Primitive? Big Boy, Jack was the most advanced fighter of his time and for eras after that. The book he wrote "Championship Fighting" contains information that still hangs true today! Read it and watch Dempsey's fights! Dempsey is a great fighter because he did it all. Fight on the inside, outside, counterpunch, and brawl. He had great speed, power, stamina, and a chin. He would knock out just about everybody in Klitschko's era. Why? Because he would beat fighters at there own game. He literally brawled with brawlers. Just look at the Firpo fight. I'm not saying Dempsey does it easily or without any hardships but he does it none the less. The fighters Klistchko fought are just too slow.
I am sure that he would be extremely successful as a fighter. For a start he would have a whole myriad of opportunities that were not available to him in his own time. These include financial backing from the time of his professional debut, multiple sanctioning bodies, more weight classes, and more elasticity in terms of which weight class he could fight in. Combine these with the fact that he was a white American fighter, with an exciting style, and it is hard to see how he misses being a sensation. To address the original question, he would have annihilated most of Klitschko’s opponents, as he was in his own time. Whether such a fighter could be developed today, is another question.
He wouldn't weigh in at the heavyweight limit if he was in his prime. He'd be a cruiserweight. I like to think that a lot of those old time heavies would be bigger today because of PEDs and modern nutrition. Dempsey could have bulked up to +210 LBs like Holyfield did.
I've just been comparing Wlad and Jack's resume's. I'm starting to think that Dempsey would have had a very tough time of it. It was Klitschko's 25th fight before he lost to Purrity and at that point his record was undefeated with virtually all stoppages down the line. Dempsey in his 25th fight was labouring to a ten round decision win over some guy called Terry Kellar, and had a loss and five draws by that time against unspectacular opposition. Wlad's next loss was at his 42nd fight against Sanders. Dempsey at that time was drawing with Willie Meehan and had managed another five draws and two losses on his record. Now I know people have derided Wlad's level of opposition, but I will take Ray Mercer, Frans Botha, Chris Byrd, Axel Schulz, Jameel McCline and Corrie Sanders any day of the week over Fireman Flynn, Willie Meehan, John Lester Johnson and Al Norton. If I recall correctly, Norton actually had a losing record at the time. Sorry guys, as exciting and explosive and advanced Dempsey was in his prime and for his time, if you transplanted him into Wlad's series of fights, he's going to come out lumped up and not looking nearly so good as you think.
The idea that Dempsey was primitive is so far off the mark it flies of complete ignorance. Langford "Dempsey is the greatest fighter I have ever seen." Sharkey "I never thought anyone could hit that hard". Sharkey stated Dempsey hit harder that Joe Louis. Fleischer who saw them all live from rinside rated Dempsey the 4th greatest hwt ever. He stated he felt Dempsey hit harder and was quicker than Joe Louis. If you know boxing technically you cannot be anything but awed by the skill demonstrated by Dempsey.
This is not really a comparison that tells us a lot. Dempsey had no real amateur background, and many of his fights are unrecorded, and he didnt even get a manager until quite far into his career. Wlad came into boxing as a former Olympic gold medallist.
Granted, but the OP asked how Jack would have done in Wlad's place. Answer: not very well. If you don't like the answer, rephrase the question. If you want to take a "prime" Dempsey and pitch him one by one against Wlad's opponents, he'd look better off, but still would probably drop a few. No doubt Dempsey improved in his later years, as can be seen by his improving record. Then again, so did Wlad.