Milan have done a really good job by taking advantage of Juventus' bad form recently. They are at the second position now after this. I find their productivity quite decent this season compared to their nearest opponents like Juventus and Bologna. The main problem is more related to Milan's defending. They have conceded 33 goals in 29 games. 
They can actually solve this problem by transferring better defenders this summer. They spent 100 million+ euros in total for transfers last summer if I'm not wrong. They should focus on their defense line more next time they make transfers. If they conceded much less goals than this now they could have been in the title race.
Sometimes, even though a team has a good performance, there are things that are missing from their view. It's possible that Milan is currently ahead of Juventus in the standings, but as you said, they have conceded so many goals and that is a serious problem, especially for a big team. Maybe even though Milan often concede goals, they have a reliable squad so this is not a problem because they can always be productive.They can actually solve this problem by transferring better defenders this summer. They spent 100 million+ euros in total for transfers last summer if I'm not wrong. They should focus on their defense line more next time they make transfers. If they conceded much less goals than this now they could have been in the title race.
But the most fearful thing is if a player experiences a decline in performance and this is caused by many factors. It seems like Milan doesn't want that to happen and they have to avoid this problem as much as possible. Well, defensive players are one of the important keys so that they don't concede too many goals because the defense also has to be strong even though their attack line is always productive. I think Pioli is also preparing a list of players they are ready to recruit at the end of the season.