I think Nico Williams has a personal reason why he still chooses to stay in Bilbao. And it seems one of them is because his brother also plays for Bilbao. In addition, as you said, Nico Williams feels he has to contribute to the club that has given him a lot of playing time to become one of the young stars in the world. I think Nico will play one or two more seasons in Bilbao before deciding to move to another club. If he can consistently maintain his performance, there will always be a big team that will want to recruit him every season.
Of course Nico Williams has his reasons for staying at Bilbao, but he should be able to consider good offers from top teams if he wants to win the title. Playing for Barcelona is not a bad choice for him, but Williams actually wants to stay for his own reasons. It's up to him what he decides, maybe it's what's best for him.
Several other teams will of course be monitoring Nico Williams' situation in Bilbao, so there is a possibility that he could go to another team in the next transfer window if he changes his mind. He is still very young and has good potential to become a top star for his position, but of course it will be difficult for him to leave Bilbao who have made his name so far.