I think Osimhen will remain in Istanbul because he likes to play there and Galatasaray wants him with them so much. The problem came with Napoli ambiguous attitudes. They don't want him in the squad and he will not play any game since he is out of Conte plans. Besides, Napoli should pay him a lot, like €24m next season while his release clause should be €75m. From what I understand, any offer above €51m is a win for Napoli! Meanwhile they refused an offer from Galatasaray of €60m so what's going on seriously with them?
Liverpool doesn't seem interested in Osimhen so far, otherwise they would make a move to grab him. The asked amount by Napoli is weird.
The one thing that makes sense for Osimhen is that he is capable of scoring a lot of goals at Turkish league, and he can stay the top goal scorer for plenty of times, and that means he will be loved. So, when you are getting paid what you want to get paid, and you succeed and you are loved, then what else do you really need? That is what he is doing and that is why he accepted this deal.
The reality is simple, we are seeing him do a great job at Galatasaray. And not like he went to Saudi league or mls neither, he is still going to play at UCL this season, one way or another. He can stay at Napoli and that's still UCL team, but at Galatasaray he will be at UCL group stages too. So he is doing alright, make sense to accept this deal.