Wasn't when Sadio Mane officially became part of Bayern Munich he predicted to be Lewandowski replacement? Lewandowski wants to leave and it will affect his performance if he stays at Bayern Munich. I don't doubt Lewandowski professionalism as a player, but if a player is forced, that's my reason to say that it will affect his performance.
Forced things will always end in bad results so it is clear in that case Lewandowski's performances will likely not be consistent anymore because he himself wants to leave the team. I think what you say is true too because a player like Sadio Mane who is being tipped as a replacement for Lewandowski will put in very good performances for Bayern Munich next season.