Are you sure nobody cares about Robert Lewandowski? So let me ask you this:
how many goals did bayern score in today's game? I'm only seeing 1 goal. now try to remember in the times of Robert Lewandowski how many they scored. you will see that bayern since losing Robert Lewandowski are not the same team, they still haven't got a good replacement for Robert Lewandowski, just see that this season bayern runs the risk of not even being able to win the bundesliga
Lewandowski is a goal machine and he can convert opportunities to goal at very high conversion rate. He grew up in Dortmund but at Bayern, he made his name as one of the best strikers in the world last 5 years. It is a mutual relationship between Lewandowski and Bayern but by letting he leaving to Barcelona, now Bayern can feel his impacts on their ability to find goals.
At least, Choupo-Moting is not a striker at same level as Lewandowski. Mane got injuries and has yet reclaimed his best performance and more important he is not a classic striker.
This issue together with some injuries make Bayern weaker and give chances to other clubs especially Dortmund. Maybe it is better for Bundesliga if they find a new champion after many years.