Xabi Alonso is not yet comparable to Pep Guardiola, but maybe he will be after he has a lot of flying time and a collection of titles. Pep Guardiola is one of the most successful coaches of this decade, while Xabi Alonso has just started his professional career as a coach. Xabi certainly has good abilities as a potential big coach in the next few seasons, but comparing him with Pep Guardiola at the moment is not appropriate.
I agree that there will be many new coaches who will emerge as top coaches for European teams. Xabi Alonson, Arteta, Xavi and several other Spanish coaches have the quality of providing competition, but all of them do not have the same path. Whether a coach is successful or not depends on the depth of the squad, financial support and several other technical things, so definitely not all of them will be successful.
If Pep Guardiola is indeed the best coach he has been successful at Bayern Munich - Barcelona - Manchester City while for Xabi Alonso he has only reached the Bundesliga and when it comes to the European cup he failed in the final although his ability in strategy is not to be doubted by never losing in the Bundesliga.
Yes it is true that a coach must be supported by many other factors, sometimes there is a desire for the coach to buy great players but the club does not allow it for other reasons or financial factors then even a great coach will find it difficult if he is in a club difficult to understand by the coach.