The reason why they sacked him was because Bayern Munich wasn't playing up to the expectations of the club board and the necessary steps they feel to tackle the issue was to actually sack him as a manager because if it comes to player, I think Bayern Munich already have all it takes when it comes to players to actually make things work so firing him and employing kompany is the next step, now it's just left for it to actually work out.
Previously Tuchel was overrated because he had a very good history when he coached Chelsea as a coach, especially the Bundesliga is not a new league for Tuchel so the expectations of him when coaching Bayern are very high.