We understand the concerns of most Galatasaray fans but we also have to realise that Osimhen is only a loan player who can leave at any time.
Manchester United might be serious about Osimhen and they've made an approach much more likely to sign him and it really depends on Napoli's decision. Galatasaray may need to buy another striker if Osimhen really leaves because Batshuayi is not one of the best strikers ever, especially with Icardi's injury, so with conditions like this, the club will definitely consider buying another striker as a step to maximize the front line to be more solid.
As mentioned before, "can leave anytime" isn't how loans work. First of all, even if they have a January exit clause, we have to realize that he can leave only during transfer windows which means that from the moment he step foot into Galatasaray until January window, he didn't had any choice but to stay, he wasn't leaving at all and that was the case until now.
The question now, is that if there is a clause that would prevent him from leaving during January. Galatasaray "could have" made an agreement which prevents it, the system exists, not saying that's what they have because I do not know much about the deal, but if you sign for a season and prevent leaving then he can't leave and that's a possible thing. Lets see though, if Osimhen wants to leave, then Galatasaray won't force him to stay, it would not make sense to force a player to stay for just five more months when they want to leave, doesn't really make sense because others won't join if they see this.