The thing I didn't understand last season was how Tuchel managed that poorly.

I mean Bayern Munich had a wonderful squad, absolutely a complete one with adding Harry Kane also to the team.
Considering Tuchel isn't actually a bad manager, it was a really surprising thing to me. Their Bundesliga performance was horrible compared to the previous seasons. I don't know maybe he was going to fix things if he was given one more year but still...
As Kompany is like a noname as a manager for most people, there is some worry about this choice. I just hope he can surprise everyone and make a good competition against Xabi Alonso unlike Tuchel's work.
Actually, Thomas Tuchel is an experienced coach, but last season he failed to give his best to the club he managed. I won't blame him completely because after all this is something that is normal for coaches to experience. And actually what makes us assume that Thomas Tuchel has had a bad performance is because this is the first season that Bayern Munich has had to end the season without a trophy and in the Bundesliga they have been able to maintain that for 11 seasons. So we see it as something significant.
Apart from that, there are other clubs that make us see Bayern Munich perform badly, namely Stuttgart and Leverkusen. These two clubs are not used to being in that position and it is even a remarkable achievement for Leverkusen who managed to record without defeat. This makes our assessment of Bayern Munich even worse, because we compare the performance of the clubs that managed to finish above them last season.