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.
You make good point that many players become stars at Real Madrid other than arriving as already famous Galácticos which shows how club and its big European games help them to shine. It is exciting to imagine Xabi Alonso completely changing team focusing on teamwork over individual stars because of his smart tactical mind. If he does this we might see some big players leave and newer less famous players who fit his system come in which would be bold change for club known for its superstars.