Yes, I also didn't think Inter could fail in the UCL. In fact, if you look at Inter's achievements in the UCL last season, Inter was able to reach the final and become runners up after being defeated by Manchester City. So Inter's failure in the last 16 of the UCL this season was really beyond my expectations. Moreover, what was surprising was that Inter lost in the round of 16. Even though in that match I was very optimistic that Inter could definitely win against Atletico Madrid. The reason is that in the first leg Inter were already superior on aggregate over Atletico Madrid. So my guess is that in the second leg Inter will definitely win and qualify for the quarter-finals. But unfortunately Inter lost to Atletico Madrid on penalties. Perhaps the biggest mistake that made Inter lose when facing Atletico Madrid in the second leg was because they defended too much. So that Atletico Madrid players have more freedom to organize attacks. And As a result, Inter conceded in the final minutes and had to undergo a penalty shootout. However, it is not a problem this season that Inter lost in the UCL. Because as @Uruhara and @W Jr discussed, Inter is currently winning in the Italian Domestic League. So Inter's defeat in the UCL can be cured by the Scudetto title which they will definitely get this season.
It's understandable that Inter Milan weren't able to scale through in their UCL encounters but their level of performance was top-notch and highly places as they remarkably did their best to stay in the contests but as always there'll always be a greater team. There's no point being disappointed in them because even though not being able to advance in the European contest, they made sure that they secure the league trophy to themselves in a grand style.
Their all time performances this season is one that should be highly ranked and commended. They did well to make a good difference this season. Their winning the trophy has given them an extra opportunity to showcase themselves in next season's UCL contest. This season, they've done well in outdoing themselves and putting up a different and better side of the usual Inter Milan we all know. They're good at what they do at the moment and hopefully, they'll do better in seasons to come too.
Comparing their performances in all seasons past up until now, it's been another remarkable height they've attained. Generally, this season has given football a different atmosphere being that teams now surprise everyone to being unpredictable on the pitch and until the final whistle is blown before a game result can be known. Just difficulty in predictions are the latest occurences but from the beginning, Inter even though outdoing themselves ensured that the initial hope of the trophy was kept high till it was won.