Age is why you'll see him declining. It's mother nature, nothing you can do about it. He's 33 years old. He still could be the best for the next couple of years, but you will see a decline.
The rematch is going to an insurmountable task for Aj. Don't forget had Joshua been permitted to continue fighting, there was still five more rounds to go. It's unlikely that Joshua can improve his stamina enough to last 12 rounds with Andy in relentless pursuit.On top of that we have to factor in a full training camp for Ruiz. Despite his physical shortcomings Andy might just be the better fighter.
the fight could happen in a few years, and that's where the problem could come. Heavyweights don't normally see decline as quick as lighter fighters probably because they're not as quick, but in the mid 30's is when you start seeing decline quite quickly. What is good about Wilder is that he keeps himself in good shape all year, which will help his longevity. Like I said personally I can't see Joshua beat Wilder now, but with heavyweight boxing you just never know.
To be fair, the outcome serves AJ and Hearn right for trying to pass Miller off as a credible defence. They got their just desserts.
I think it would be in Joshua's best interest to steer clear of the top heavyweights for the next year or so, at least until he shows he has some stamina and has some time to regain some form of confidence. Another fight with Ruiz with the same type of performance and outcome will put an end to his career. Right now many believe that it was a fluke win for Ruiz and that is about all Joshua has going for him.