it's totally sensible to work this way if you have no money.
if you want to change the squad and dont have money, you will have to buy a replacement for someone (in this case hopefully sissoko) you will sell or plan to sell.
the only way to get money for other transfers is, buy a replacement that is cheaper than what you get for the player you want to sell (momo).
so instead of having some sissoko warming the bench and getting paid good, get a mediocre one that earns less which will free funds to spend elsewhere.
on top of this its surely better the youngsters to compete with some pazienza for a backup place, then with a high paid sissoko. since the quality difference is not that big the youngster might actually be more motivated (because of the probability of taking pazienzas place as backup).
it's funny how someone with a brain could even think that we should buy stars for millions of € just to replace backup players in positions we are already well supplied with.
i can understand if someone would blast the purchase of a pazienza-level fullback, but getting a cheap replacement for a player that is not needed and can generate funds by selling him is totally sensible ...
we will have pirlo, melo, marchisio, pazienza for the central midfield spots ... thats totally ok for a team only having two domestic competitions to play in next season. an expensive backup in form of sissoko is just not needed. we actually improved our starting line up while paying no transfer fees and being able to get money by selling sissoko ...