This season, the big clubs in Europe have tried to take him with a lot of interest, but if he does not show such performance in the next season, then the big clubs will not show much interest in taking him.
Nico Williams has been offered by many clubs and if he accepts the offer now he can sign a long-term deal with those clubs and not have to worry about his career. If he decides to stay in Bilbao, he must improve his performance and play the whole season.
It's like a gamble because if he can improve his performance, he may receive offers from top clubs, but if his performance is worse than at present, his demand will decrease. There is still enough time so hopefully he will make the decision that is best for his career.
He made the decision to stay at Athletic Bilbao, there were several factors for this, and one of the main ones was that the Europa League final would be held in Bilbao. And his brother, who also plays for Bilbao, also played a role in this, because to some extent he managed to persuade Niko to stay in the team and try to win this tournament, I think for Athletic Bilbao this is important, but I am not sure that for Williams this is the right decision, because he already had the opportunity to play in a stronger club and a more prestigious tournament.