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I agree, Nico is more of a winger, Lewandowski is a striker, a goal machine, pure scorer. So, the difference is huge and that won't be the replacement. I honestly still expect Raphinha to leave, Nico is young, he can do better with time and he needs to be seen as a player not for right now, but for the future of the team. If they can also get a good young striker instead of Lewandowski, they would be at a level where they can keep nearly the whole team for 10 years.
This isn't easy but it can be done for them. Nico can be bought, and Raphinha could be sold, and Lewandowski is on his last year of contract, so they just need to find someone who can be great for them at striker position, that could make them a lot better on the long run.
Rather than sacrificing Raphinha, Barcelona would be better off releasing Ferran Torres, Ansu Fati or Pau Victor to give Nico Williams more space. Lewandowski is in his final year, and Hansi Flick seems to still rely heavily on him up front. The addition of Nico Williams would certainly add variety to Barcelona attack next season, and his signing is clearly a long term investment. I think the situation Barcelona will face is similar to what happened to Real Madrid last season, when they signed Mbappe and turned him into a center forward.