I think Hearn is confident that AJ would win, less confident that it would make $100m.

I reckon that Hearn wants to get Wilder against his second string Whyte, for lesser millions. If Wilder wins by KO it ramps up the interest in Wilder AJ. If Wilder gets beat (most probable) depending on the performance it could still ramp up either a rematch or straight to AJ. Either way it puts Wilder among Brit viewers and may well improve his standing with more savvy US viewers, most of whom think he's shit. I guess its maximising your assets.