Oda writes fights based on narrative, not powerlevel. If he wants Shiryu vs Zoro to happen in a way that instills the same sense of superiority King had against Zoro, he will write both characters at a power level that is necessary to tell such story whether Shiryu is 42 or 70 years old, after all Urouge got significantly stronger in his 40s too lol.
Also if post-Wano Zoro is already beyond King-level, then there's no doubt left about Shiryu not being a top tier.