If chelsea can succeed to sign him it will be a great deal for the club, their attack will be more stronger and they will comfortably challenge for the premier league title.
That is if he does not get injured.
If I am Victor Osimhen, I will be very selective of the club I play for and will not just want to move to any club because it is an English team. He is already very welcomed in Napoli and has no complain about the team, the coach, the management of the club and the supporters of the team in Napoli, so he may not be too keen on leaving to just any club that he knows will be difficult for him to win trophies in. He has won one very good trophy winning the Italian league, it is normal for him to feel the urge to want to win more and will want a club that he can win in. In terms of the competitions that Osimhen will have an opportunity to win next season, Napoli will play in more competitions than Chelsea will, so staying in Napoli is also a good choice.
I would understand Osimhen if he would like to play for a team competing in the Champions League by next season. Chelsea are rich maybe but they had a horrible season the last. Therefore they aren't going to be a part of this tournament. Osimhen might not want to be a part of a rebuild phase at Chelsea as he will need to be really patient to compete in the Champions League once again.
He has been improving his performance significantly and it is important for him to continue to shine in big competitions as much as possible. Technically the EPL is the most competitive European league but Napoli must be still attracting to him due to the Champions League ticket.
Maybe we would see Bayern Munich going for Osimhen if they are prepared to sacrifice an amount more than €120m most probably.