While I agree that even if a champion is unchanged, the game can change around said champ and make them suddenly an appealing pick.
However, this is just not the case with Shaco. Shaco isn't particularly interested in Cinderhulk, the item that started the jungle buzz in 5.5. He doesn't benefit from the Athene's buff unless for some reason you build him AP, which is dumb unless its an ARAM. Soooo....exactly what game change are you referring to that made Shaco nerf-worthy? If anything the fact that he falls off makes him LESS viable because tank junglers scale extraordinarily well into the late game and Shaco doesn't hardly at all. Rek'Sai is still good, Vi is still good, Lee Sin is good, meaning there are also early game junglers that can prevent Shaco and his lanes from snowballing. All in all, I agree with what you are saying, but Shaco does not need to be nerfed. He is at most a tertiary beneficiary of various changes, and even that is stretching it.
Also, quick note on Sejuani: most people already said Cinderhulk might push her to OP status, and she was already seeing LCS play before Cinderhulk's introduction. So I don't think people thought she was trash-tier at all.