I agree with you that the outcome of this match was completely unexpected. None of us expected such a result. At the same time I want to say that Inter Milan were very unlucky in this match. Otherwise, the team would not have lost against Atletico Madrid.
Inter Milan players missed all 3 penalties against Atletico Madrid. The important thing is that 3 goals were missed by the best three players in Inter Milan's squad. Hard to accept that Martinez, Sanchez and Klaassen will miss penalties. All three are excellent players, but all three have missed. It is nothing but bad luck.
Inter Milan should now focus on Serie A. However, the Serie A title will easily be won by the Inter Milan team. The Inter Milan team is ahead of AC Milan in the second place of the table by a big gap of 16 points.
In the penalty round, something bad can always turn into a surprise like what Atletico Madrid got and Inter Milan should have been more worthy of qualifying for the last 8 but luck was not on Inter Milan's side.
One small mistake prevented Inter from qualifying for the last 8 when competing at home, they couldn't get a bigger aggregate score, they only got 1 aggregate score and that was a difficult condition and underestimated Atletico Madrid, a big mistake and now it has been proven that Inter Milan just make a small mistake that has bad consequences.
This failure is not the end of everything because it can get first place to get the series A title but this title will not really make the supporters happier.
After this, Inter will not have anything to do except focus on maintaining its position to continue adding large numbers of points.