Have you forgotten that Napoli has a new coach this season, and yet they were able to win the Scudetto this season? So I believe Inter Milan signing a new coach won't stop them from winning the Scudetto next season if they play well. It is true that Napoli is serious in the transfer window, recruiting players to get ready for next season and possibly defend their title next season, but I still believe Inter Milan has been in good form in the Serie A league for years, and they have a good squad depth that can rival Napoli for the Scudetto next season.
I agree with this because even though Inter Milan is managed by a new, inexperienced coach, their chances of competing for the Scudetto are still quite high. But to see who has the best chance, we might have to wait until mid-season because sometimes teams can be much better with a new coach, or vice versa. Napoli is indeed more serious in the transfer market, bringing in several players because they know next season will be much tougher.
They will be playing in the Champions League, so relying on a number of players from last season will not give them a great chance of achieving consistent performance because they will play more matches next season, so they will need to add a number of new players to be much more consistent.