Dean Henderson is a first-choice keeper. Why on earth would he sign and compete with another first-choice keeper in De Gea. Backups are backups for a reason they are not good enough to displace the first choice, especially a specialised position like GK were its detrimental to rotate and you want someone permanent between the sticks to gel and communicate with the back four.
De Gea will be gone by next summer I think, it's the only thing that makes sense. Then you've got Henderson first choice keeper tied down for another 5 years still. 120k doesn't look too bad then.
One thing I really am looking forward to is some authority and communication between goalkeeper and backline, De Gea really lacks any kind of commanding presence in the box. Which for me makes a massive difference to your defence, been saying this for ages, De Gea doesn't give any confidence in the box at all, rarely claims anything.