Maybe give them a bit longer than half a season before you write them off. As you say, the point is that they develop into better players we can sell in 2-3 years - they aren't there when we buy them. So you have to allow that development over time.
Now ideally we would do that with a couple at a time, not most of a squad, but we were where we were. There are already signs most of our summer signings are improving as they settle. Let's see how they look by the end of the season before judging.