Despite Real Madrid's weak defense this season, they have won the Champions League and La Liga titles. The main reason is that their midfield is very strong. At the same time, Benzema was playing well entire the season.
Real Madrid defenders are not really strong, but they aren't weak. They can't win 2 top titles (UCL & La Liga) if their defenders are weak. It is true that Real Madrid midfielders and attackers are strong, but they can't win many matches the whole season if their defense is weak. All lines must be strong enough to perform consistently till the end of the season.
AC Milan is probably the champion after 10 long years. And so next season they will want to increase their strength and win the title again. AC Milan has not had a regular scorer this season. Giroud and Ibra are certainly good players, but now Milan needs young talent. Inter Milan and Juventus continue their efforts to increase their strength. If AC Milan can't bring in new talented players, they will be left behind in next season's title fight.
You are right. AC Milan needs to improve their attacking lines. They cannot rely on Ibrahimovic and Giroud anymore since these strikers are getting old. Especially Ibra, he may spend more time on the bench in the next season. I think AC Milan can try to sign a striker like Dybala or Lukaku. They are quite strong strikers with a great capability in scoring.