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.
All big clubs feel pressure of playing many matches and all Real Madrid, PSG and Chelsea did play many matches last season. Differences among them in total played matches are not too much as PSG won Champions League, Real Madrid went far there, and Chelsea won Conference League. PSG won against Real Madrid in this FIFA Club World Cup too so if a shock defeat in a final match is because of exhaustion, it's kind of unreasonable. PSG actually prepared for the final badly and they lost it without any reasonable sorry while Chelsea are truly deserved this winning and the trophy.
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.
It's an accident for PSG like how Inter Milan got an accident and big loss in UEFA Champions League final match. Considering it as an accident does not make this winning of Chelsea less impressive as Chelsea truly played well and this big win for them is because of their excellent performance, not only by bad performance of PSG.