Well, I barely seen any improvement on our defenders and they had a guy who is one of the all time greatest CB too
True story, but Koeman played in a totally different time and also in a different role (libero/"sweeper") than today's CBs. He hasn't made the transition into today's football and it could be seen in his tactical approach. He might has come across as a commander, but I doubt his tactical preparation of players was anything near as substantial as Xavi's will be.
I expect Xavi to be way more demanding to his players when it comes to tactical guidance. During the game, he is way more active on the sideline than guys like EV or Koeman and more in the mold of coaches like Simeone, Conte, Pep, Klopp when it comes to ingame coaching.
I expect our midfielders to learn a lot from Xavi. He is one of the biggest football brains of the last decade, has been coached by Pep and had an awareness on the field that rarely gets matched. Not gonna say Xavi will ever get nearly as successful as Pep, but I'm pretty sure players will develop into better players. Busquets wouldn't be the footballer he has become without Pep's guidance and mentioned guys are not less talented.
I don't think selling players his footballing idea will be a problem for Xavi. Much more likely for me that his stubbornness on tiki-taka and obsession with passing will hold him back.