If Wilder had just took that $100m fight with AJ when they both had the belt instead of losing to Fury 3 times, rate both if they fought and retired then? AJ would obviously have defeated Wilder based on what we know now but they both would have made a lot more money than with their fights after their losses.
Indeed, that's the fight that we missed in the post Klitschko era. The Lewis/Bowe of the 2010s. Obviously in that case we would still have want to see how a fight with Fury would have unfolded.
As you say, Joshua would obviously have won... And he'd have got mega credit for it if Wilder retired straight afterwards, much as Fury did (as much as it's aged like milk since) immediately afterwards. It wouldn't be that deserved given what Wilder hadn't proved at the time, but without those humiliatingly clear losses with Parker and Zhang, plenty of people would still be blind to just how average he was.