Isco is not similar to Xavi. He's more straight forward and offensive. Thiago is way closer to Xavi than him. Plus, I think people are expecting too much from Gündogan. He's no doubt a world class player, but he plays in a completely different system than ours. We don't have a guarantee that Gündogan will succeed here, we saw many players fail at our club in the last few years who are great in other clubs. The best way to replace Xavi is to promote players within our own ranks as we know what we'll get.
I doubt they don't see anything in the likes of Thiago as he was convincing in the last few games he played, he was even one of the best players on the pitch a few times. Roberto and Rafinha have barely played for the first team and you can't judge them on their performances for the B team. In the end, who would have thought that the likes of Busquets, Tello and Pedro would become the players they are today?