It’s a fair point — I think the choice is either:
1. Take the lowball sum this summer, right now; or
2. Let him go on loan where I’m fairly confident he will do at least OK enough to remind everyone he is a PL level fullback for a club a lot lower down the table (it’s funny because 20th) and the fee would be close to around £16-20m, even in his last year of contract. But like you say, there is the risk he just comes back and sits on his arse for that final season.
However, the club do have the option to extend for a year. Not quite sure how those work, and if it can only be activated in his last season before it expires or if we can activate it at the start of the 23/24 season to try and get a bigger fee.