Stamina is all this comes down to. Dempsey cannot get Ali out early, take KO and TKO out of the equation because they aren't factors. Dempsey would have a lot trouble setting up his left hook against an opponent who fights unlike anyone he'd encountered in the heavyweight division. Ali can easily dance around Dempsey til he's gassed or absorb the punishment to bide time and allow Dempsey to gas himself. Rope-a-dope would be too effective against guys like Dempsey and Marciano who were all too gracious to expend 5 hard punches to land a single bomb. Ali will outlast Dempsey every single time, I just cannot foresee Dempsey ever winning this.
I will say this much: Reconstitute them to prime every fight, let them fight 10 times, 12 rounds each time, and Ali's not winning them all. (IMHO, of course.)