Oshimen had one or two good seasons and that was it from him and when there were interested parties like Chelsea who were prepared to pay over €100 million, he should have been sold. Today he will never be worth that amount and no one will pay such a particularly high amount. There are many players who play very well for a short time and reach their best quickly, but they also fall just as quickly.
Not only Chelsea, PSG actually already made an offer above €100 million. One of the media stated that PSG offered €130 million for Osimhen. But Napoli rejected the offer because they want to sell him at €150 million. Actually, Osimhen's market value is only around €100 million but Napoli seems to not selling him at low fees. Napoli may try to keep their main attackers for the next season (Osimhen and Kvaratskhelia). However, there are more and more rumors related to these players. The latest news I heard from Napoli that they are trying to keep one of those attackers at least.
By the way, the market value of Osimhen in the future will depend on his performance in the next team. It will also be crucial for his career. Considering this matter, I don't expect him to move to Chelsea. I think it will be better to move to PSG because he must have a chance for winning more trophies there. In Chelsea, he may win 0 trophy in the next few seasons.