The problem is that stupid law in Italy which prevents teams from building their own stadium. We waited 10 years to finally be able to build it. To get a permit it takes ages. But the other things you mentioned all stands. Italians can only blame themselves.
Its not just the leaders, the major reasons lie elsewere
1) Stadiums. When we were ahead of the curve, it was fine the city owned the stadiums. When the profit of a privately owned stadium started to increase exponentially, england shot past us with all their teams investing in a privately owned stadium. Italian teams wanted to do the same, but legal issues and citycouncils blocked that for more then 10 years. This was the main reasin why epl got so rich. Not the oil, that came later.
2) Player wages. Italy has a serious tax issue. When wages became more important on the budged, and the television money stopped rising so much each year, we suffered the worst.
those are main reasons.
3) calciopoli, but not for juve. Since calciopoli, milan partially and juve entirely vanished from the scene for a year, and returned weaker. Plenty of smaller teams tried to get into Championsleague.
Because the traditional sides were weaker, they had less dept, and dropped more points from european football. This had as effect that since 2006 most smaller sides didnt care about EL, but tried a push for that 4th CL spot instead
Thanks to serie a just getting 3 CL spots. Juve is back. Milan seems to be on the way back, and lazio/inter seemb to be fighting for that 3th spot with roma likely to follow in next year. Teams are forced to do well in EL for that 4th spot. They need it, we dont.