I think the reason for the drop in Osimhen’s market value will be because of the issue surrounding his move to Galatasaray and the kind of deal Napoli is offering. I see no other reason for the drop because despite the circumstances around his move he has being performing well for Galatasaray. It is good news that Napoli are finally deciding to reduce his price tag, this will make it easy for interested clubs to make a deal for his services in the summer.
It is normal if Osimhen price is decreasing because he still doesn't win any trophy with his current team. Osimhen is also not in the top leagues in Europe anymore. So, I can understand if Osimhen price is no longer as expensive as in the previous season. However, I don't see any decline in Osimhen performance, he is still one of the best striker in the world. Because the price is already around €75m, I think it is more possible for Osimhen to leave Napoli in the next season. I hope he will move to Arsenal, PSG, or Real Madrid.