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.
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.