He still has the highest value relative to how much we actually need him, on this squad. If you sell him to City for 80M and replace him with Rugani, then what are you losing?
The caveat, of course, is that you have to invest in the attack and take pressure off the defence. I'm confident that Rugani-Caldara will be good enough in the future to hold the back line if you have a great team in front.
I would like to shift our focus from relying on defenders and you can start here. We've seen countless times now that Bonucci is not good enough to stop the absolute best attackers like a 2006 Cannavarro. But then again, nowadays, who is? I can't really blame him to be honest, it's much more difficult to defend even in the 10 years since 2006.
But that still leaves the opportunity to cash in and switch the focus of the team. And it's something I would think about.
What defenders are even worth the money we can get for Bonucci? There are none. Barca was winning with Pique. Madrid won with Ramos and Varane who just jog around the pitch. They are quality CBs but nothing extraordinary.
Building around defenders is like saying you have the best coast guard team, they save 95% of the people who fall into the dangerous river. While other teams just build a dam.