Separate games mean nothing. Maybe PSG was exhausted by the previous games plus everyone predicted an easy win for PSG and this led to the burnout of the PSG players. But even if Chelsea is really stronger than the best team of the season at the moment, this does not automatically mean good prospects ahead. Bookmakers quote Chelsea's chances of winning the title as 1 to 13. I think you will agree that against the background of City, which went into crisis, the fresh champion and Arsenal, which have passed their peak, this looks like a very weak quote.
I disagree with you. PSG was not exhausted. They're tactically outplayed by Chelsea. They're doing high press, which was leaving a lot of space for Chelsea players. Hakimi gets marked by Neto. Enzo, Caicedo and James overload the mid, which made them have no room to forward the ball to their attackers.
Even Reece who played as mid very discipline in moving deep to help Chelsea in defense. However, i do agree Chelsea's success in winning CWC doesn't guarantee their future, and their odds literally said it all.
But I think you have often seen in a favorite-underdog pair such a situation that the favorite plays a suboptimal strategy, and the underdog plays a counter-strategy to the favorite's strategy, but the favorite still wins. So you can't explain everything by tactics. The game lasted 90 minutes and Enrique had time to change something, but the game ended not 3-1 or 3-2 but 3-0. I think the reasons are more complex than just winning because of one tactic.