BigCat was clearly one of their best players the final two months of the year. You can question whether the cost (2nd) was too much to give up. But letting him go would be a massive mistake. If he's not brought back, I'm going to lose a lot of faith in John tbh.
If BigCat is not brought back, my thoughts on the decision to not bring him back will depend on the overall direction indicated by the totality of decisions made this offseason. One potential decision that we do not know the reasoning behind seems like a rather naive reason to lose faith in someone.
Suppose he is not brought back and the totality of other decisions indicates that 2024 is a reset year with a focus towards competing in the following years. In that case, I will be happy with the decision to let him sign elsewhere. Conversely, suppose that he is not brought back, he signs elsewhere for a reasonable contract, and the totality of other decisions made by the front office indicates that we are pushing for a competitive season in 2024. In that case, I might question the decision to let him sign elsewhere. And, even then, I am not in the building with access to the thought processes and reasoning of the coaches, scouts, and front office (for all I know, Mike tells John that BigCat doesn't fit what he wants to do and he'd rather roll with player x).
What we gave up in trade to get BigCat is irrelevant to the decision to re-sign or let him walk. Weighing sunk costs in the decision-making process is a practice of the uneducated and unintelligent. I am certain that JS and his crew are much too intelligent to fall victim to the sunk cost fallacy.