I remember when Napoli got kicked out of the Champions League against AC Milan in April in the quarterfinals, they weren't able to get back into their best shape again. They lacked motivation, but due to the huge advantage they already had over all other teams, it didn't really matter.
Inter Milan is out of the Coppa Italia, they are now out of the Champions League and while their goal was to play the nearly perfect season and they were indeed on a good way, they are now left with not much to play for. I am saying this because they are too far ahead in Serie A to still lose it. It is 14 points and I think it will be hard for them to only have this lead in the table to be looking forward to play for. It' s a totally different thing if they were still in the CL knowing that there are highlights waiting for them until the end of the season. Now it is over for them with any highlights.
Shit happens. Inter is king in Seria A this season, but not in UCL, because UCL is for champions from various leagues in which every club is very strong. This is why you will see that a club can be strong in his domestic league, but will not be able to go far in UCL. So many clubs like that.
However, been eliminated in the Champions League does not mean that they should be unhappy with such lost. They still have the trophy to win and become the defending champion of the league next season. They will only need to work on themselves and prepare for next season so that they don't get kicked out so early from the champions league.
Nono, you are oversimplifying things here obviously. Inter Milan has by far been one of the best teams in the Champions League last season and that was a time when they weren't performing outstandingly in Serie A like they are doing now. They were doing ok last season, but it was nothing special. However, in the Champions League they blew everyone's mind because Manchester City has had big problems beating them and they needed a deflected ball in the end to win that game with a single goal scored while Inter Milan had several chances to score the equalizer. This season Inter is again an amazing team, but the Champions League is a competition where it is not always the best team that wins the title, but the one that was most efficient with their chances and maybe had some luck as well. I think that Atletico Madrid would more often lose against Inter than vice versa if they played a dozen times against each other. But this is the CL and Atletico made it in two games.