You don't think Johnson could have smothered and frustrated him? Although you may have a point as Dempsey didn't need much space to get those hooks off.
I think Johnson does just that, & that's why Dempsey doesn't blow him out in two or three (something he had the power to accomplish, if he caught Johnson just right). Look at it from the other point of view. Johnson, flatly, never dreamed of a whirling dervish like Dempsey in all his years. A faster, more savage, electric puncher, he could not have had a nightmare about. I think Dempsey's aggression, far from playing into Johnson's defensive hands, overwhelms him early. Most of Johnson's opposition weren't even serious combination punchers, & he had no trouble picking them off. This would be different. Dempsey presents under-rated handspeed, good foot placement & positioning, a lot of precise punching, devastating power from either side, &, of course, preternatural finishing instincts. I think he finishes Johnson.
Absolute prime Dempsey 1919-1920 or so might have a chance against some versions of Johnson...........who had a much longer prime then Dempsey BTW. Dempsey had a hard time against smallish, boxer type Light Heavyweights. I have a hard time believing he'd match up well against a larger, stronger, better boxer type Heavyweight.
Johnson's men were mostly smaller than he was, & typically threw one or two punches in a sitting. Watch Johnson sit back & counter, or parry the punches. It's no preparation at all for a demon like Dempsey. I doubt, too, Johnson's ability to absorb all the punches I think he can't stop.
Big men weren't exactly Dempsey's weakness, incidentally --- not that there is anything more than a small size difference here.
No, you are right. Guys who could parry his attack were. Guys who could do that and counter at the same time were an even bigger problem.
They usually were nimbler on their feet than the flat-footed Johnson, though. The guys who escaped for any length of time --- Gibbons, Tunney, for instance (the latter of whom only fought old, sickly & inactive versions of Dempsey) got moving & kept moving. In-&-out, side-to-side, & without pause. Johnson fought much more stationary than those men, & relied upon quality counter-punching (again, against a backdrop of few, if any, combinations from his adversaries) & superior size & skill to get the job done. I have trouble seeing him tie a rampaging Dempsey up successfully indefinitely. For a time, yes --- Johnson was clever, wily, & could punch enough to give Dempsey pause --- but I don't think he can keep it together forever. Dempsey solves the puzzle, & bangs a game, but offensively out-gunned, Johnson on out of there.