Ac milan losing to LFC in a close game is 1 thing
The surprise is ac milan finished bottom of the group and lost at home to what is close to a reserve team, when the reserve team had nothing to play for and you had it all on the line
They finished bottom of the group only because Atletico blatantly cheated them in their first meeting, Milan were leading, one red card (undeserved) and one penalty (again undeserved) later, Atletico won it 2-1. Without that result, we'd be seeing a role reversal with Milan 2nd and Atletico 4th. And it's their first CL campaign in almost a decade. Yesterday, they also had a few players missing. Their captain, Kjaer is out for the season, Calabria, their primary RB was also out. Rebic and Leao, two of their primary attacking outlets, were also out injured.
We are currently one of the Top 3 teams in the world. When Milan were at their peak (90s), even Man United, who were dominating the PL could never have competed with them. That's the cycle of life.