Napoli's decline in performance was completely sudden. They were consistently performing well. But suddenly Napoli conceded a 4-0 defeat against Milan in Serie A. This was a completely unexpected result. And in several matches after that, we saw erratic performances from Napoli. Napoli's performance against AC Milan was also poor in the Champions League match as well. And so Napoli failed to qualify for the semi-finals.
Both Inter Milan and AC Milan seem equally likely to win. The performances of these two teams have been erratic in Serie A this season as well. So there is no way to say for sure which team will qualify for the finals.
The game will be played, and one team will undoubtedly qualify for the finals, albeit we are unsure which one due to their outstanding individual performances. The final game is the real deal. I don't think any of these clubs, assuming they make it to the final, will be able to face either of the other two teams in the semifinals, Real Madrid or Manchester City. I know champions league finals are usually intense and fierce, but I doubt this one will be given the teams who will face each other in the finals.