Even though Tottenham Hotspur are known for sacking their coaches who take them to a final or win a trophy for them, I think Ange Postecoglou's case is totally different. Tottenham Hotspur finished in 17th position in the Premier League table, and Levy is afraid that Ange Postecoglou will likely not perform better or help Tottenham Hotspur compete in the Premier League next season. So, for Levy to avoid any poor performance from Tottenham Hotspur next season, he decides to relieve Ange Postecoglou of his duties at Tottenham Hotspur so that he can get another coach who can help Tottenham Hotspur compete in the Premier League and UEFA Champions League.
Next season Tottenham are going to play in the Champions League, a competition which is clearly above their level so, in my opinion, the best case scenario for them is to reach the first round of the knock-out stage, that's it. I can't imagine Tottenham at the quarter finals or semifinals. At the same time I expect Tottenham to perform way better in the Premier League, this season they clearly focused on the Europa League, and they won the best.
Since the new format is supposed to give everyone weaker and stronger opponents, it is not impossible for Tottenham to reach that stage and it is also not impossible for them to get an opponent they can beat in the final 16 round. Last season there were teams like Feyenoord, Brugge, Lille, Eindhoven. In general I agree though that there can#t be a lot of expectations as to what they can achieve there.
Still I don't think their team is horribly bad. They still have several great players and maybe get an upgrade during the transfer period. Surviving the league round with a few wins would flush some well needed money into their pockets. They could be a great chance for players who don't feel ready yet to go to City or Liverpool, but still want to play in the Champions League. Similar to Cole Palmer who couldn't get much done when he played for Manchester City, but then thrived at Chelsea.