Understand strength of feeling for Cook, however if we don't go up this year then surely he will have failed to show the premier league quality that would get him a move back next year. I know he can't single handedly get us promoted but it will be hard for him make a case for lower PL move if he isn't captaining a defense that gets lots of clean sheets in a winning team.
Thus regardless of the outcome of this year he is likely to want/prefer a new contract, whatever division we are in. In contrast, if we get promoted I can see the club wanting to upgrade the defensive line so we are stronger than last year, which might be at his expense. It could be quite mercenary.
Therefore, if his pay is most important to him, then moving now is his best option. I hope he stays but let's not overlook the need for a right sided CB to replace him next year as first choice if we get the success for which we hope.