Kind of peculiar that Allegri would declare him the future leader of the team. That's a bit stupid to do if you're contemplating selling him.
Flat out, the only reason given our current financial situation to sell these two is if we want to spend obscene amounts to replace them.
We have about 100-150MM in transfer spend capacity. Add Sandro + Bonucci, you're looking at 230-280 million in NET transfer spend capacity. Assuming Lemina is sold, that puts you at 250-300MM to buy a new LB and new RB because we do have 5 other centre backs to use if we play a back four, including increasing Rugani's role and using Caldara. There is no way buying two replacements will cost more than 60-70MM. So then??? what are we going to do with 170-230MM in net transfer capacity?