I agree with you because it is still too fast for Xabi Alonso to become a big club coach like Liverpool and Bayern Munich, especially if he fell Real Madrid in the next season. Xabi Alonso has not been very tested in the coach and may take time for him to develop a career in a club full of history. What Xabi Alonso needs to do with Leverkusen to a higher level in the Bundesliga so that his name will be one of the coaches discussed.
Zidane might be one of the priority if Ancelotti stopped to become a Real Madrid coach and he had given something good for Real Madrid. If at this time asked who is more suitable to be a Real Madrid coach then I agree more if Zidane is their coach.
Why you doubt him?
I think it is okay if Xabi Alonso becomes the coach of Liverpool, Bayern Munich, or Real Madrid.
Don't you see how impressive the stats made by Xabi Alonso when he is managing Bayern Leverkusen this season?
It is an average team only but Xabi Alonso can build them to be an unbeaten team. It is enough to prove the quality or capability of Xabi Alonso as a professional coach.
When Zidane became Real Madrid coach, he is also not tested enough. But he could succeed with Real Madrid although many people doubt him. It also can happen with Xabi Alonso, he can succeed with Liverpool, Bayern Munich, or Real Madrid although some people assume it is too early to manage a top team.
For the the next coach of Real Madrid, it depends on the situation. If Zidane has an interest to manage Real Madrid again, he will be the number 1 priority. But he has no interest to manage Real Madrid, Xabi Alonso should be the priority to be the next coach of Real Madrid.