It is a big win and also good news for Galatasaray but I am not sure it is good for Osimhen with talent he owns. This decision of binding his future with Galatasaray will not allowing him playing for any big clubs in Europe in near future. He is no longer too young to move to any big club after his 30s and it's doubtful if such transfer happens, he will be able to perform well after his 30s.
There is no doubt that it is big win for Galatasaray, they finally can have Osimhen permanently. Paying around €75 million seems not very expensive price considering the great skills of Osimhen. Osimhen has proven his ability as a top striker in the last season, it gained the interest of Galatasaray board to have him as a permanent player. Well, I think it is quite good for Osimhen's career. Being a permanent player in Galatasaray will guarantee him to play regularly as the main player there. If he returns to his best form anymore, it is not impossible that other top teams in Europe to try signing him again in the next few years. As far as I know, his contract with Galatasaray will be until 2027. There is still enough time to play with other top teams in Europe. SPL teams will also try to negotiate with him if he can maintain good performance.