Lewandowski has been a very important player for Bayern Munich scoring goals and helping them win trophies but at this stage of his career he has decided to leave the club for a new challenge elsewhere, I think Bayern should respect his decision and find suitable admirers and deal that will see him move, a deal that will be satisfying to all parties. It would be better to make a deal and let him leave than keeping him against his wishes which will only make him unhappy and that wouldn't be good for the team's performance.
I agree that it is not going to be an insanely well prepared great situation, of course Lewa will be sorely missed if he leaves. However, you can't rely all of your success into just one player, if he leaves then you should be able to get someone to make it work.
I mean they already have great players, so they should be fine if he leaves, at the worst case put Mane there and you are done. This isn't about Lewandowski not being needed or anything, of course he is great, but this is about ANY player in any club, you can't just hope that one player makes or breaks you. If you rely on single player, then you will have to pay 400 million like PSG had to do to stay afloat.