I don't know but i would not really say that Bayern Munich has underperformed that much in the last years. Yes they have only reached the quarterfinals in the last 3 seasons but we should not forget, that in the Champions League it is not enough to be the better team in general you also always need the right timing and a bit of luck on your side.
Actually, at the beginning of the season, their performance was quite good and consistent. They also lead the Bundesliga with quite a distance from the others, as well as in the UCL where their performance is still good even though Lewandowski is no longer around. However, the longer their performance declined, even in the Bundesliga, they were overtaken by Dortmund. And what's more, their victory and their championship in the Bundesliga was decided by their last match which luckily Dortmund couldn't be optimal. This is really different from previous seasons where Bayern Munich really dominated. And sadly, Mane didn't look so optimal at the club last season. So, they really need something new so that their team can be even more optimal this season