Buying star players and building great squad depth is part of the process for a team that wants to win the title. PSG did it right and they have gone through that stage, but that doesn't necessarily make them successful in the Champions League.
So far PSG have dominated Ligue 1 and that is part of their success in building squad depth so far. The Champions League is a difficult competition, so it is not easy to win even if the squad is good. However, PSG qualified and fought in the Champions League final several seasons ago even though in the end they lost 1-0 to Bayern Munich. It was part of an experience they must not repeat if they qualify for the finals next season.
But they sell Messi, Neymar, Verratti and Ramos are the sign they're not really want to win the Champions League. They only sign mediocre players in this season and they didn't sign a top coach, so I don't expect they will go anywhere in this season.
Mental could also become one of the factor, since they can't become a champion, it might make them think they're not really deserved in this league and they feel inferior.
The way PSG buys star players like that I think is not entirely appropriate because after all, the players PSG buys are old players. Unlike about Real Madrid and Manchester City do, they have star players who have great potential at a productive age. Not only that, Real Madrid and Manchester City also managed to develop the potential of the players they had to be very good so that ya, Real Madrid and Manchester City also succeeded in the Champion League and not only in the domestic league like PSG. Therefore, basically PSG needs young players who have great potential and at the same time, PSG must have good management as well to form a better squad, and they cannot just rely on Mbappe.