That's not a grounds, that's an agenda.
Even if it came down to them fighting, Shanks wouldn't "mid-diff" a Yonko known for having one of if not the highest defenses, let alone being a walking juggernaut, who has been a Yonko for years and maintained his position since. Shanks is strong, but not that strong to "mid-diff" an Emperor of the Sea. That's not a power creep, that's just overestimating someone's abilities as a whole.