Dele's problem isn't about his quality on the ball or mentality, rather it's the fact that injuries have taken a toll on his body.
Remember Daniel Sturridge? That's what's going on - it just can't do the same things it used to, regardless of how clever or technically brilliant he could be.
The way the payment is structured, however, makes it reasonable for Everton, as in, the penny will drop very quickly that Dele just doesn't have it anymore and will stop getting played, eventually being phased out before the fee goes up significantly.
He could have the odd good game here or there but how valuable is that? You don't rise up the table relying on sporadic displays of form.
Spurs will never ever see those 40 million.