Inter Milan was able to take advantage of a situation where tough rival teams such as Juventus, AC Milan and Napoli were in a slump. However, I still admit that Inter are very strong and they deserve the Scudetto. In fact, Juventus is also very strong and has the potential to disrupt Inter's smooth progress. It's just that the old lady has to deal with so many cases that she loses concentration and allows her rivals to rejoice at her condition.
For next season, the competition in Serie A looks like it will be much more exciting because several teams are also making improvements to their respective teams. It is very possible that Juventus will return as the main contender for the Scudetto, and I am sure they will rise again under Motta's control.
Inter Milan work hard for it, they deserved it and luckily they got it. AC Milan underperformance did not surprised me like the other two teams that you mentioned. Although Juventus performance since last two seasons was very awful and not convincing enough to rate them among contenders but Napoli was outstanding last season and at early stages of just concluded season. It was actually shocking how things turn around and they couldn't maintain their standards thereby making it easier for Inter Milan. Inter got the opportunity and they utilize it appropriately.
As for Juventus, I don't see them coming back stronger or better than they were last season because they are yet to restrategize their squad and it's very necessary. You can not continue to work with same equipment over and over again and expect different results. Aside the couching crews, the squad itself needs overhaul replacement and adjustment since some of their players no longer show sign of commitment.