Regardless of who is to blame. With your reasoning - we are asking our best goalkeeper who has over the last 7-8 years been our best or at least close to our best performer to take an insane paycut - not because we have to because of finances, not because he is overpaid in terms the wages we have at our club, but because you think its his market value. If we give DeGea £150.000 a week - we would demote him from one of our best performers, to one of the least paid first-team regulars. The only way to do something like this (if you have a TINY understanding of human behaviour) - is if you want to chase DDG out the door. Because that is what will happen - no sane person will accept a 60% paycut - unless maybe to save the club from bankrupcty as a gesture of goodwill.
But what you don't seem to grasp - is that if there are just 2 clubs with money competing for a 32 year DDG, "the market value" thing doesn't really count. Simply because since there is no transfer fee involved, clubs can afford to offer him a bit more to win a bidding war. But maybe you are right, maybe they will only offer him say £180.000 and a 4-year contract- of course they will likely sweeten that deal by a huge signing on fee. Let's for sake of argument that amount is £15 million - which isn't unlikely. Then that will be the equivalent to £75-80.000 extra a week over his contract period - so that renders your "market value"-theory rather void. Players who sign on free transfer don't get market value in their contracts. When Sol Campbell joined Arsenal from Spurs, the latter refused to give him £100.000 a week. Arsenal reportedly give him that and roughly £8-10 million in signing-on-fee. Which over his 5 year contract added another £30-40.000 a week.
You also make it sound impossible that anyone would give him above market value - market value is what anyone is willing to pay for him, not what similar goalkeepers get paid. The fact is - in Spain, there is a club DDG knows really well. Jan Oblak is their best paid player - almost on the same wages as DDG. Let's theoretically say that Oblak hands in a transfer request - Atletico sell him for £70 million and are short one goalkeeper. And they get DDG back for £15 million in signing on fee, and give him £300.000 a week - still £50.000 less than Oblak reportedly has. So when you say "it's really that simple" - you really over-simplify that. It's never that simple.
We gave him a contract of that amount a few years ago (which was too much granted) - I am sure we can get him to accept a slightly lower wage in his new contract- but your reasoning of an insane wage-reduction is only valid if your idea is to allow DDG to leave on a free. Because that is what will happen, and then we have to spend £50 million to replace him with a goalkeeper who may or not be just as good.