Not to mention that Thuram is a top earner along with Lautaro, Hakan and Barella at 6 mil net and Thuram has a 5 year contract. If it is up to me, I would give a raise to Bisseck and Frattesi.
https://www.capology.com/club/inter-milan/salaries/
Yes, he is an established team player now, with great scoring and assist record. Scudetto and promise for next year great season or two. Why would he go anywhere and start from 0?
Well, if you compare Buchanan and Darmian, Darmian is better and more experienced. Buchanan is too raw still. I'm not saying do not play Buchanan, but as a sub.