It won't be the reason not to sell.
If the financial offer appeals to the manager and directors, then he will go.
Cooper saved us several points last season with his saves, and it will be difficult, if not impossible, to find an equally skilled replacement. Your assertion that a 30 something could replace him seems a little naive.
He is still young and for Cooper to go, he would have to have a reasonable chance of gaining a first team place at a higher division club, otherwise the advance in his career path will be interrupted, and, although it would mean he would be turning down financial incentives to move, he would be better off staying until the right move came along for him career-wise. He has many years ahead of him and he has to choose carefully what moves he can now make in order to further his career later on.