You're not listening, I'd suggest you to study Johnson opponents a bit more. He didn't fight many small men.
He fought a lot of shorter opponents who had absolutely no skills when compared with Jack Dempsey. What Johnson was good at would not really help him with a skilled fighter who could move. It doesn't matter if it was Dempsey, Ali, or Tyson etc. Johnson's skill set was great at neutralizing any (no matter how big or small in height OR weight) flat footed opponent who stood in front of him. That wasn't Jack Dempsey's way.
Any all time great Heavyweight who could move side to side with hand speed and savage power would have been a nightmare for Johnson's style. You can't clinch what you can't catch. Just like you can't hit what you can't see.
Since when Denver Ed Martin, Sam McVea, Sandy Ferguson, Klondike Haynes, James Jeffries, Frank Moran and tons of big guys were small? You base your whole picture of Johnson on Burns and Ketchel fights, which are not representative to Johnson competition. Not to mention that Johnson beat more skilled opponents than Dempsey. Yet he beat Joe Jeannette, who was a mover. He beat Gardner who was mobile fighter. He beat Denver Ed Martin, who was big mobile jabber. He beat plenty of black fighters known for their movement. If you really believe that Johnson could only clinch stationary opponent, then you have big gaps on your knowledge about him.
I like Dempsey's speed and power in this one. I think sooner or later he cracks Johnson a good one and then gets him out of there. JJ would have to slow the tempo down and turn the fight into a mauling affair. If he gets through the early rounds and dulls Dempsey's edge, he would probably win a fight over a longer distance. I don't think he does. Dempsey seems much faster, more accurate and heavy-handed than JJ on film.
Jack Johnson inside grab, clinching constantly would frustrate any body attack from dempsey. Johnson has stylistical advantage
Langford was 5'6 Dempsey was 6'1 But Johnson didn't fight much smaller men right? It doesn't matter how skilled a 5'6 midget is when the guy they are facing is much bigger. Dempsey was a tall guy with skills and attributes that Johnson's own skill set doesn't match up with very well. You are trying to compare a grape with an orange.
I already mentioned Denver Ed Martin who wasn't small but you ignored this part. Sam Langford was a midget who was one of the best heavyweights ever. He was arguably better than Dempsey at HW.