well if he's dead-set on not renewing, of course it makes sense to sell him rather than let him go on a free!
It would only make sense if they had a replacement (which they dont), he's vital to our midfield and having him play there next season should be worth more than selling him for 20-30m. But on the other hand he's Toni fucking Kroos and I wouldnt excactly expect a Lewandowski-like performance from him with the contract running out and no World Cup / Euros next summer.. yeah you are right they should sell him.