If Mane plays better and if Bayern find a good striker to replace Lewandowski, Bayern Munich will become a stronger club in a new season and they will dominate Bundesliga again. Their chance in Champions League will not be small because in a new season, many big clubs will be in transition phases like Real Madrid.
When talking about Mane, his performance is declining so that it is very influential in a team game because even one player's mistake can have a significant impact.
To be able to replace Lewandowski, at least Bayern have to spend a larger amount of money and this is an obstacle for several teams that cannot make transfers properly.
But I agree with you that if Bayern had better substitutes it is possible that they could do in the Champions League.
Sadio Mané is surely on decline after having exit from Liverpool with Bayern Munich is not suitable place for him just because of this now Bayern Munich management looking for new striker which keep things in better shape which is important for them after end of this bad season even they won Bundesliga, but their performance is on decline which is surely matter of concern for them.
There were rumors about few players which are under their radar with possibly Sadio Mané is also going back to England but how things will take turn in coming days is not confirmed because now Bayern Munich is also looking for better end with good results in Champions League which are not happening for the last few years.