Sometimes that's what I worry about from Milan, if they want good players they have to sell players who don't contribute much. But this will certainly be understood and every club must also do the same thing for the good of the team and the attitude that must be taken by Milan must be firm for all of its players, without exception. Samuel Chukwueze is a talented player and he's shown it through this season very well and it's no wonder he's getting the attention of Milan.
Some of these names seem difficult to develop at Milan and I think it should be from Milan that the players understand what they want for next season. Even so, even if Milan is not good enough, who knows in other teams it can improve for Origi, De Ketelaere and Yacine Adli. So I think they have to determine their future apart from hoping for Milan.
Clubs must always make sacrifices, whether it's selling players who are actually playing well but can't maximize their ability to bring in players they think will be able to play better than the players they're going to sell. It's normal, of course, but it must really be considered carefully, I mean when they sell players they have to bring in players who can really play better than the players they sell, because if new players they just play mediocre it's the same, lying is better to retain players who are on the current roster.
Maybe one of the options if they still don't want to sell the players they have is to let their players go on loan so they can develop and hopefully when they return their abilities are much better as you said.