That's the right thing to do, their current manager is not giving good results to AC Milan and has already disappointed their fans. With the change of manager, hopefully there will be an achievement that AC Milan will get or at least they will be more competitive in Serie A to get the Serie A title.
Whoever is chosen later to replace Marco Giampaolo, that is the best choice for AC Milan. And you are right, that this change is the best step taken by the AC Milan, because they can not wait any longer to be able to get up and return to compete with the positive path.
Changing managers tends to do very little and it's my suggestion that they keep the manager for a little longer, they are yet to finish the first half of the season so I think it's too early when it comes to the talk of sacking the manager, things might change mid season.