It only makes sense if Chelsea can win - otherwise Osimhen will only ruin his career, especially if he stays there for long. If he needs to win, it's a mistake to move to Chelsea. He needs to choose another club that can most likely at least win their league and who are decent in the Champions League. It's hard to say exactly which club would suit him and even more so which club would be able to accommodate him, but definitely not Chelsea
Osimhen as said by their president in Dicember is already sold but we dont know to whom.
A reason on why we dont know yet is because it may go to EUA and there the market session will start in 1 week.
It may also ends at PSG, they needs to replace Mbappè somehow even if they are not spending so much money lately.
So TLDR, Napoli will have the money from Osimhen and they can spend for Lukaku.