Very true! If PSG still had Mbappe they would have been much more deadly and very difficult to play against but let is also think it this way because in life some things that may seem crazy at the time have to happen for a change to come, if Mbappe did not leave PSG wouldn’t have made the signings they made.
Mbappe wanted to manage PSG. He didn't listen to the PSG coaches. And the PSG management gave Mbappe a lot of support. I would say that PSG was better off leaving PSG. PSG was overly dependent on Mbappe. Luis Enrique is trying to gradually reduce this dependence. And he has succeeded.
Dembele has been performing very well this season. But PSG is not only dependent on Dembele. And this is their main strength. That is why PSG is able to perform consistently well. It is true that PSG's attack would have been stronger if Mbappe had been with PSG. However, I doubt that such a strong bond would have developed between PSG's players.