I think the "he must have something" arguments come from a disconnect between Saint Shanks's feats and his portrayal. imo, Saint Shanks outright has the best feats of any character alive. From splitting the sky with WB, stopping the war, making admirals shit their pants with CoC from miles away.
Not even kaido has feats like this. However, Saint Shanks isn't WSSM, nor is he WSC or WSM.
Here's how I think it could be reconciled
- Saint Shanks is a failed Roger. He might have been PK, but sacrificed it for luffy when he gave up his arm.
- Saint Shanks is going to become the WSM(or close) via a massive power up, which will also change his fighting style.
In regards to #1, Saint Shanks has the greatest haki in OP, he just doesn't have the raw strength that Kaido and Mihawk do that make them stronger. This shows that Saint Shanks had the potential to be PK, but he gave it up for Luffy.