Why Sato would have destroyed Miyagi. (Cobra Kai Spoilers)
I think Sato would have defeated Miyagi if they were to fight. Here are the reasons.
1. Miyagi left Okinawa when he was still young. Sato on the other hand stayed and apparently maintained a good relationship with Miyagi's father. Thus, he would have trained in the more advanced Miyagi-do techniques Chozen used against Daniel. Chozen provided his own speculation as to why Daniel wasn't taught the paralyzing techniques, but it is possible Miyagi never got a chance to learn them in the first place. Sato learned more, and thus passed on that learning to Chozen, who easily defeated Daniel in their rematch.
2. Sato remained fit, where as Miyagi relied purely on skill. When facing Sato, fitness and Sato's size advantages would be insurmountable.
3. Drive. Sato have been waiting and training for decades to face Miyagi. Whereas Mr. Miyagi always believed he would talk his way out of fighting, so he never maintained the edge needed to win the fight. Sato had every reason to defeat Miyagi, whereas Miyagi felt he had no reason to fight. Give Sato the psychological edge.
Here are the main reasons why Miyagi would not win.
1. Size matters when both competitors are skilled. Miyagi is considerably smaller than Sato and would lose.
2. Miyagi had no drive to defeat Sato and probably couldn't bring himself to kill his friend. Sato on the other hand, had no reservations.
3. Miyagi trained in Miyagi-do for less time than Sato and never learned the advanced techniques.