That's probably the best summary of what he could offer and I think board had the same ideas giving him contract extension.
Selling him to finance a decent replacement doesn't make sense as he already is a decent player with the advantage of being versatile.
He was given an extension when Gazidis and Sven were still here so that might not be all that relevant anymore. Also you don’t sell him to finance a replacement you sell him to increase the transfer budget and help rebuild the squad. £20m for Chambers can go towards a CB or winger who can improve the first team. We have enough squad depth in the areas Chambers can play.