Rudi Garcia is a safe choice for Napoli as he already spent some seasons in Serie A with AS Roma. He failed to win Scudetto with Roma but at least he can help them to end at the second position which is a good position for Napoli too. It is unrealistic if Napoli think that they can win more Scudetto in coming years.
I will strongly believe that Garcia will not be as good as Spalletti in providing strategy to this team and will only perform mediocre like last season.
You are right there is a chance to stay Osimhen is only performing well when he is playing for Napoli and there is even the chance to say Osimhen can have a good performance just when he is playing with the system in Spalletti's team. This means he can't have this performance in other teams and that's why there are not many offers for him at this price, because we already saw similar cases when the players left their teams. Núñez was a good example when he left Benfica to Liverpool.