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What Nico Williams did is something most players wouldn't do. He showed true dedication. Athletic Bilbao is one of the best teams in La Liga, but they can never stay in the title race until the end. The biggest reason for this is their policy of only playing with Basque players. If they were part of the global transfer market, they definitely would've won a few more titles in the modern era. But in today's globalized football world, trying to surpass Barcelona and Real Madrid with only Basque players is almost impossible. And when both of those teams stumble, Atletico Madrid steps up.
If Nico Williams stays for the full 10 years of his contract, there's a high chance he'll never win a title. But I don't think that really matters to him.
Just like you said it, "Basque players", Nico Williams may not be unconnected with such identity, hence his dedication to the club than going anywhere else including Barcelona where he can earn higher pay and also win trophies. Just imagine him signing a new contract that keeps him in Athletic Bilbao until June 2035. No player with the intentions of leaving the club will pen down such long contract.
It's just unfortunate he has such strong attachment with a mid table club as Athletic Bilbao which makes it very much unrealistic that he might finish with any trophy as you said. In today's football it's the big spenders clubs that dictates the rotation of trophy and if your club is not spending on buying and building the team with top talents then it's a no no...