The decline of Italian football has been occurring even before Calciopoli, by the time Calciopoli was exposed to the media. It was a week or two before the 2006 UEFA Champions League final between FC Barcelona and Arsenal. By that point, it had been 6 years into the 21st century and by then, 10 different sides participated in the 6 UCL finals played in the 21st century by that point. Out of 10 sides, only 3 of them were Italian and 2 different clubs as Milan played in 03 and 05. The fact that people blame the downfall of Italian football mostly on Calciopoli is ridiculous, and to make the stats even worse, 20 different sides played in the semi finals of the 21st century, out of 20 sides, only 4 were Italian.
The downfall really occurred before 2006, it was happening you can say as early as 1999, as it was the first time an Italian team didn’t play in the European Cup final since 1991.