I think he takes a paycut to try to win with stevens
He already got plenty of money. He doesn't seem like the money hungry. Lets be honest, at this point he wants to win and with his coach, they have unfinished business.. Money doesn't buy that
Why should he take a pay cut if he has a great year? The Celtics are not in a position where, if he takes a pay cut it would help them sign or re-sign this or that player. Are you assuming that he will get offers far above what the Celtics think he is worth? If they don't think he's worth that much, then I am guessing he didn't have a good year.
The decision will be primarily based on what location he can win a title and play a main role. I think he probably realizes he's not going to be the best player on a title team, but he could be the second or third. Is that going to happen here, or will Brown and Tatum push him down the hierarchy? What about the big man question, has that been resolved by that point? How likely is it to be resolved in the future?
I would guess all situations being equal he stays, but if there's a clear cut amazing situation out there then he might leave. Horford left for a better basketball and contender situation, though the money helped too.