Even if Chelsea buys Osimhen, their performance won't change that much. Look at any new signings they have made recently, most of them have not been able to perform up to their expectations. Mudryk, Caicedo are prime examples. They haven't done much of anything. Cole Palmer, Connor Gallagher, and others have carried this club in the last season. Without them, the results would have been much worse for Chelsea. Osimhen may or may not perform like he has been performing in Napoli. On the other hand, they are about to sell one of their best performers of last season, Connor Gallagher to Atletico Madrid. If that happens then Joao Felix will also be joining Chelsea, who also flopped a little when he was at Chelsea.
But I think by the end of the transfer season, Osimhen will find a new home. I don’t think he is happy in Napoli and he will be pushing Napoli officials to do the deals.
You have made some points regarding why Osimhehen presence likely won't change anything but we still don't have to conclude, asin whether he will perform well or not, in some cases, it isn’t always the fault of the players to not have a expected performance but also the pattern of the coach and the position at which those players are being featured in a game, we all saw what happened during the Frank Lampards era as a coach, most of the players he bought them, didn't perform but Thomas Tuchel used them properly and we saw how greatly the players perforemd.