I don't know why Chelsea management fired Tuchel. It was one of their wrong decisions. Tuchel is a very experienced coach. And with Tuchel's help, Chelsea won the Champions League with a mediocre squad. If Tuchel had been Chelsea's coach, he might have been able to stabilize this team. Tuchel was fired probably because of coach Tuchel's problems with the management. And despite several changes of coach since then, Chelsea's performance has deteriorated rather than improved. Chelsea have now become a relatively weak team. We don't know when we will see consistent performance from this team again. Chelsea's performance is unlikely to stabilize this season.
Undoubtedly, Tuchel helped Chelsea in winning the champions league but things started getting worse after that. Every Chelsea fan clamored for his sack back then. I don't see it as wrong decision by the management because it was very necessary as at then and moreover, I don't think that incident has any connection with their current misfortune. Chelsea have appointed at least two or more couch since after the departure of Tuchel and should not be used to as an excuse or to justify whatever condition they are into presently.
Chelsea current squad is full of average and mediocre players. The couch has his lapses but Chelsea problem now is more beyond the couch and any couch will suffer the same problem until there's overhaul change in the squad. Even during the era of Tuchel, Chelsea performance was never impressive but were lucky to be winning matches including the champions league. Nothing change since then but while they still maintain the poor performance, the luck of winning matches is not longer there. I must admit that Tuchel reign was far better than this current reign of Pottechino. The return of Lampard was the greatest mistake to ever happened to Chelsea.
Personally I think Tuchel would have been able to improve Chelsea's performance if given more time. After Tuchel became the coach of Bayern Munich, Bayern Munich performed poorly in several matches. But gradually he stabilized the team. And now Munich has become a strong team again.
Chelsea's squad is full of talented players. Their squad is still very strong in the Premier League. Still they are not playing well. The team performed well against Tottenham and City. From this it can be understood that they have the ability to do well even against big teams. But Chelsea's performance is erratic due to tactical weaknesses. I would say this is due to the inexperience of the coach and management.