Manchester United are done and they will be hard pressed to return to their former glory. Sir Alex Ferguson was the last coach to win the title for Manchester United and that happened 10 seasons ago. We know that Manchester United is the only team that has won 20 Premier League titles so far, they are only 1 title ahead of Liverpool and 7 titles ahead of Arsenal. Manchester City can overtake anyone if they can maintain their best form over the next 10 to 15 seasons, but I think the story will never be the same because there will be players who come and go in that time frame.
After the retirement of Sir Alex Ferguson Manchester United hired several coaches which include David Moyes, Ryan Giggs, Louis van Gaal and José Mourinho. Others include Ole Gunnar Solskjaer, Michael Carrick, Ralf Rangnick and now Erik ten Hag. The most successful of them all is Mourinho who won the Community Shield, the Carabao Cup and the Europa League and also took the club to finish second in the EPL. Moyes and van Gaal were good coaches that the club expected much from but they ended up achieving little. Will I say that the club has not been lucky in choosing the right coach? But ten Hag still have the chance to prove his worth.
Chelsea's victory over Tottenham cannot be used as a complete reference that they have marked a turning point in performance, I agree with you that Man City is difficult to beat even though Pochettino squad is high in confidence. It's true that Chelsea is still an elite team in the EPL, we can't just call it bad because they've been inconsistent all this time, it's clear they can create a lot of surprises. However, if the Blues succeed in beating Man City at Stamford Bridge, then Pep squad strength this season is questionable.
If Chelsea play well then the best result they can achieve is a draw, it would take a bit of a miracle to make Pep Guardiola disappointed with his team. However, we cannot possibly forget that Man City have also lost so far, so anything can happen.
Chelsea got the maximum three points in the game against Tottenham, which is most important. But that match cannot be used as evidence that Chelsea has improved. Any team could win a team that has lost two players and that Chelsea just maximised their opportunity. The real test is the game against Manchester City. Mauricio Pochettino and the players should prove that they have regained form by winning the current EPL champions.