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.