The contract situation isn't the only factor at play though. We're also operating within a tight budget in a summer where we need to sign quite a few players. If we overspend on Mount, that has direct consequences on which striker we can sign later in the window.
In the grand scheme I'm sure we can afford to pay it but it's not as simple as saying that it's only £5m. It's actually a £20m increase on the original offer if we did cave, in a situation where we hold all the cards. If we're prepared to take that kind of hit on every transfer this summer just because the selling team is being awkward, that accumulates very quickly. If we signed 5 players and did that for all of them, that's £25m just by virtue of being poor negotiators.
If Chelsea don't play ball after we amend the structure of the payments, we'd be absolutely right to move on. The player can then see we've exhausted all options to make the deal happen and so will the manager. Even if Mount is a key target for ten Hag, he'll be acutely aware that we're operating within a restricted budget. We can't continue to push those thresholds for the sake of one player. This is how most normal clubs operate, we're just so used to paying stupid fees for players that our fanbase has become desensitised to it.