Not that he's not in great shape for his age, because of course he is, but what do you mean his body has held up remarkably well? You make it sound like he's put himself through it when he works a safer, slower, more grounded style than probably 95% of wrestlers. If anything, he should wrestle until he's 60.
That being said, I don't have much to weigh in on this topic as I'm not a WWE viewer. I do want to echo Chris' thoughts on how overrated I think he's become as a reaction to him being shit on so hard for the first several years.
And independently, I'll add the following critique: Great pro, always comfortable on the mic and in the ring, reliable as hell, and a fantastic ambassador... He's really good at his job. I would even say there have been times where I felt bad for him about his booking, considering how dependable and versatile he is. He isn't fucking Ricky Steamboat though. He's not Dusty Rhodes. In 20 years, he has about three matches and one promo that people reference as being great, so I don't think anyone is missing any hidden magic ability that he has.
Lastly, I want to acknowledge that the previous paragraph was critical and will no doubt be read by some as evidence of how much people try to put Miz down, but let me be clear that my criticism isn't towards The Miz, it's towards the public sentiment on his legacy and how people evaluate him as a worker.