not every decision is swayed by money. what poch would be paid at utd would be balanced by the increased pressure. he's probably loaded anyway, why would he want that extra pressure at his age? he'd be far better commiting to spurs for the next decade on the premise we keep our best players as we don't need to sell.
Its a fallacy to think that just because you don't need to sell that you won't have to. No one wants to sell their best players and most teams don't have to for financial reasons. They do it because if they don't they will either have an unhappy player on their books who perhaps doesn't perform or could even be disruptive. They also don't want a 50m player running down their contract and leaving on a free. There are also clubs that are stepping stones to bigger and better things and Spurs are certainly not at the top of that pile. Why would youth players or exciting players with dreams of going to the best sides in the world go to a club who decides that they don't have to sell so if you perform well then you will not be allowed to move onto bigger and better things.