we may be too early to speculate but expecting Osimhen to be a replacement for Mbappe seems to be very impossible and it would only be PSG dream to get Osimhen while he is still under contract with Napoli and on the one hand, Osimhen knows how PSG treats Mbappe, who is always in trouble when he wants to move to other teams to improve his career and maybe Osimhen is also thinking about his future career, perhaps if he moves to PSG he will only be a League 1 star, not a UCL star.
Actually PSG performed quite well in the UCL. They made it to the semifinals and that means they are among the elite European clubs. But the problem is that Ligue 1 is often referred to as a farmers' league so not many media highlight Ligue 1. Honestly in my area if there are several matches in one night people will choose to watch the EPL, La Liga or Serie A rather than the Bundesliga or Ligue 1. If Osimhen wants to be a UCL star I think PSG is still very viable because they will almost always play in the UCL every season. But your performance in the league will not be highlighted by many media and this will affect Osimhen's career.
It would be better for Osimhen to play at PSG if he only stays at Napoli, because at PSG at least Osimhen can guarantee to get a trophy every season. Meanwhile, if Osimhen only plays at Napoli, the reality is that to being able to compete in the top four is also difficult for Napoli. Also, PSG recruiting Osimhen is not completely to replace Mbappe's role, because at least Enrique will also adapt the right strategy and tactics to the players in his squad. Moreover, PSG is still making great efforts to be successful in the Champions League and thus, of course playing at PSG will be better because of its continued efforts to make better improvements.