It was the Spanish giants who started it in the 60s and kept it going until the 80s. Milan had already won the European cup twice and the league a dozen times even before Berlusconi's arrival but they were destined to shine again with a number of quality homegrown talents they had(Baresi, Maldini, Costacurta, Galli, Tassotti, Albertini) etc. Half of the team were already there before Berlusconi so Milan just needed a competent owner willing to spend to put them back on track and where they belong.
Yeah but all those things happened in the late 60s.
Milan were in a horrible state before Berlusconi bought them. Twice relegated in the 80s - first for the Totonero scandal in 1979/1980 they finished 3rd but relegated and later for being just shit in the league when they finished 14th in season 1981/1982.
Then their team standings were
1983/1984 - 8th
1984/1985 - 6th
1985/1986 - 7th
These positions look decent but at that time there were only 16 teams in the league
Then Berlusconi buys Milan in February of 1986
1986/1987 - 6th Broke transfer record for Donadoni 8m€ also bought Borgonovo for 2m€ and a few others for an undisclosed fee - Massaro, Galli, Bonetti...
1987/1988 - 1st Bought Gullit for almost 7m€ and van Basten for 1.15m€ and a few others AGAIN for an undisclosed fee - Ancelotti, Bianchi, Colombo...
...
Milan just in the 90s spent over 200m on new players or around 130 netspend
Inter's spending was even more absurd with 350mil€ on new players. Overall Serie A in the 90s was spending way too much money on players and then you got clubs that got fecked due to finances - Parma, Lazio, Fiorentina...