Only Mbappe is not enough to make a strong team and PSG need more than him to have a strong attacking system and a strong team need more player resources. Asensio and Dembele came as replacements for Messi and Neymar but those players won't contribute too much for PSG proactive defense at their frontline. It is not important in matches with weak opponents but is very important when facing against biggest opponents in Europe. Losing a ball and don't chase opponent players to stop their counter attacking at the frontline, it will cause nightmare for PSG defenders and goalkeepers.
Isn't it true that when there were many star players at PSG, their results in the UCL were not good? So maybe making Mbappe the main center will be easier to manage. The current PSG coach is quite experienced and knows how to sail in the UCL competition. Asensio and Dembele don't need to be substitutes for Messi and Neymar because being themselves and playing with their abilities will be much better. There's no need to burden yourself just because of Messi and Neymar's previous positions. If PSG wants to make changes then the solution is to move on and be confident in their abilities without having to rely on the legacy of Messi or Neymar.
When Lionel Messi was in the PSG team, the coach of the team could not play well by maintaining his positioning. Currently the team is not in the best condition. It remains to be seen how far the team can progress in the Champions League.