It will be absolutely asinine to sell him for 15-20m upcoming season unless we can use that for a absolutely quality CB to take his place. Tuanzebe, Jones, Rojo, Baily have been unreliable at best and the Lindelof/Maguire partnership has been decent but, not one that gives anyone confidence of the bedrock of a league/CL winning team.
This past season was a somewhat throwaway in some sense to rebuild but, now we've got to show some progress, that means we have to be able to at the very least rotate and also put some competition for Lindelof. I like Baily but, he is not reliable both in terms of health and quality at times and an Injury to either Maguire / Lindelof and we are in trouble for a reliable CB to hold down the fort.