The comparison isn’t between Shanks and Rayleigh though, it’s between Shanks and Mihawk. I also doubt Shanks isn’t stronger than Rayleigh in his prime. Shanks is on the level of WB, Kaido, and BM, people that were Roger’s contemporaries. Shanks is probably stronger than Rayleigh was.
The issue is not founded on a limit, it’s about the pattern of portrayal through the story.
Shanks is not on their level tbh.
Holds the same rank? Yes no doubt .
Same strength nope.
As we saw the spoiler info the marine notes Shanks crew unlike the rest of Yonko crews whose commanders are not even mentioned.
So Shanks is the youngest Yonko(status) and have the strongest crew most probably.
Pattern of portrayal will change right in Wano ,955 already gave us the indication of Luffy and Zoro being the only ones (shown in panel) with the tools to hurt and defeat Kaido.(What will the excuse be at the moment? Oda is failing ? Or is doing justice to the rest of his charactes beside Luffy?)
Like for example you don't expect Zoro leaving the island as the strongest swordman of the island (above King ,red scarbads,etc) .Or you think he will leave being weaker than them?
Remember Luffy and Zoro will be legends that's already mentioned in the beggining of the series PKand WSS .