Nico Williams has the ability to perform well. We have seen better performances from the yamal-williams pair in the Spanish squad. However, Williams will not be in Barcelona's starting XI this season. However, Hansi Flick will probably use Nico Williams as a replacement for Lewandowski. I will not have much expectation from WIlliams this season.
They combined with each other well in Spanish national team but if the transfer cost is too high, Barcelona must carefully consider about it together with their on going financial crisis for making a due diligent decision. I believe that it's better for their finance if they ignore a transfer of Nico Williams as it costs them many things like transfer fee, and high salary for this young player.
With current available players, Barcelona already managed to win La Liga and participated in UEFA Champions League semi finals and were not too far from the final match. Personally they don't have to bet in a transfer of Nico Williams while no sure results for a next season like La Liga title and Champions League title. If they win none of La Liga and Champions League titles next season, their transfer bet on Williams will harm their finance a lot only within one season.
Nico Williams is not a replacement for Lewandowski. Those two players have nothing in commong and they play not the same football at all. Lewandowski played with less speed, but he puts more emphasis on positioning and one of his advantages is a strong instinct for game situations that can't easily be anticipated.
Nico Williams far more often plays 1v1 against defenders and he likes to speed up with the ball and dominate on the wing.
This is a problem that many coaches would like to have. Flick has Raphinha and Williams available and if he wants to replace Lewandowski, I think he would rather take Raphinha and let him play in the center of their attack line. Nico Williams won't play in the middle. Maybe Flick will try it out, but I don't think so.