Even though I can say you are sort of right about self-confidence, I also agree with lixer. Galatasaray built terrible roster, mainly because they think last year's team is functioning and good enough so they only got some players to fix specific positions. But it failed. Only because players from last year are literally terrible. Fenerbahçe did it better renewing squad.
Unfortunately Fatih Terim has always been known to get his kind of players and he was never good at roster engineering. I do not remember too many times he found a gem for cheap that turned out to be a great player, he always spends money on local players who are not good enough and pay a lot for them and almost always turns out to be not enough of a player. Right now Galatasaray have players that we could have seen from anatolian clubs usually, or even second division (named first ironically).
So, with a new management, new manager, and new players, in a few years there is nothing stopping them from becoming the best team again, but it will take a while before that happens.