Alonso has all the star players that any coach will need in order for him to win the any competition that they take part in. Real Madrid is used to signing star players since onset and that's why you see that they can maintain their standard. All the players in Madrid are all star players and none of them is an average player. Which means that Alonso needs to know how to manage these players in order to bring out the best in them.
Contrary to the popular belief, Real Madrid has actually signed very few star players in the past few seasons. Most of them turned out to be "stars" while at Real Madrid thanks to those repeated European magical moments
Xabi has been given greenlight to organize the club the way he wants and with that I am very sure some big names are going to be left unhappy and will consequently leave the club. Real Madrid's recent signings can give a clue on how Xabi wants things to be. A club that plays as a team and not one that relies on one player or two for individual brilliance.