Wemby from last season really shows signs of greatness already. But your argument has some weight though, not all number 1 pick became a great player, there are some that are really bust because of injuries or wasn't able to adjust to NBA playing style.
But for Cooper, it seems that this kid is ready, but it's not like Wemby or Lebron during their first year. So yeah, maybe he needs a couple of years to developed and reach his prime before we can truly see if he is really that number 1 pick that everyone is talking about him.
I would say he is not going to be a bust, but he is not going to be a superstar neither. I think he is going to be just a good player ,and there is no shame in that. For example, Dame Lillard had a great career right? Could you say he is not a good player? Of course not, he was a great player and he played greatly all his career, I would say we are at the back end of his career now but that doesn't mean we are going to end up with anyone saying he was bad.
I would say that is the level of career Cooper probably will have, not some Giannis Jokic level player, but could be good enough to be part of something bigger, or just carry his team wherever he can. These are all important stuff and could make him be a lot better over course of long time.