Losing Lewandowski is indeed a big loss for Bayern Munich but I don't think this can be considered a mistake because Lewandowski really wanted to leave Bayern Munich and if Bayern did everything they could to keep Lewandowski at Munich, I think it can affect Lewandowski's performance and motivation in playing football (it could be that Bayern Munich's performance is much worse than it is now). So when an agreement has been reached, all Bayern Munich needs to do is find a player to replace Lewandowski.
Obviously it's not a mistake but the player's desire to leave and leave him so Lewandowski is thinking about his future with another club, I'm sure this will soon find a solution for Bayern Munich who are looking for a replacement according to the criteria needed but I think Sadio Mane is also good only it just takes a little more time to adjust this in my opinion.
But I also expect the transfer window in the winter to hear news about Harry Kane who will be recruited but that will all be proven later after the transfer window opens for sure Bayern Munich will look for the best replacement.