Look at it from the club's perspective. Right now, we get around 100m. Maybe a little more or a little less. If we are pretty sure he won't renew, the club has 3 options.
Sell now and replace, probably get ripped off.
Sell now and don't replace, use what we have
Keep him for a year, potentially destabilising the all-important squad harmony and losing 30m.
With option 3, maybe he decides to sign. Or maybe he has a great season and we still get a similar amount to now. But those are by no means certain, Especially if he's sat on the bench.