While osimhen played a huge role in Napoli trophy win i don't think we can say that they won't be able to win the serie A because there is no team that are ahead of others in the serie A so there are a few who can win the title such as inter juve milan and napoli, even teams like roma and lazio might have a small chance this is how close the teams are in term of level i think as long as napoli team secure a decent striker they have a fair chance to be champion again.
Next season is likely to be even more exciting in terms of the title race in Serie A, even though the Napoli team can secure their best striker. Because if the Napoli coach is not so consistent in making his tactics or strategy for the next season, then Napoli will have a little difficulty competing with his rivals. Now everyone has seen how Inter, Juventus, Milan, and Lazio did last season so they can all make even bigger surprises next season in Serie A.
In essence, it will always be difficult to defend the title so yes, it will not be easy for Napoli to come back strong next season in the title race. Although Osimhen and his new coach can make a good performance for Napoli, but I believe it is still not enough to make Napoli consistent throughout the season. Because I believe, that next season maybe Inter and Juventus will compete fiercely in the race for the title but even so, I realize there is no guarantee that these predictions will come true. But one thing I still believe is, it will be difficult to defend the title and that will be experienced by Napoli next season.