Let's see, 2018 was the summer we bought Ronaldo. At that point we were a team that was close to the top, so we added Ronaldo to make that difference. That summer we had:
Szczesny
MDS Benatia Chiellini AS
Cuads Khedira Pjanic Matuidi
Dybala Higuain
Buffon
Licht Caldara Rugani Spina
DC Bentancur Sturaro Asamoah
Berna Mandzu
So we let Buffon, Asa (to inter of all teams), Licht walk. We got Cancelo so we had the Licht departure covered. Asa was replaced already by Spinazzola so that's also OK. We paid 14M for a backup keeper even though we got the last two for free who were more proven arguably (Neto, Szczesny). Khedira was kinda finished so instead of going for a reinforcement we got a freebie in Can (who was expensive too at 16M and 5M wages), who was rotation level for us. But then we spent 35M on a CB (Bonucci) while we let inter get De Vrij for free. We failed to sell Higuain in the same transaction that liberated Milan from Bonucci and his heavy wages and gave them a (at that time) promising young domestic CB to pair with Romagnoli, so that's another failure
Szczesny
Cancelo Bonucci Chiellini AS
Can Pjanic Matuidi
Dybala Mandzukic Ronaldo
Perin
MDS Benatia Rugani Spina
Cuadrado Khedira Bentancur DC
Berna Kean
We lost to Ajax due to injuries and fatigue. We lost Benatia and replaced him with a Caceres who hasn't played in a while, so when Chiellini went down it left us with a not ready Rugani to step in. In the summer we got De Ligt and Demiral so that's a good fix there, but the midfield was still bad. Ramsey and Rabiot for free were decent additions, if we had a starting midfield in place and needed depth. But we overpaid in wages and will have trouble selling now. We decided to ruin the one area we actually fixed successfully, so we sold both Cancelo and Spinazzola. Danilo was a expensive replacement himself and right now even when he's good, he looks the best away from the RB position in midfield or at CB. We got Pellegrini as well, but he might never play here. We also spent some 30M on Romero, another guy that was never to play here. Then we loaned out our 14M backup keeper and brought back Buffon, who we just let walk a summer before. Higuain was back as well and Kean was plusvalenzaed. Then we told Mandzu he wont play anymore when the season begun, instead of selling him before. And we sold Can in January because we couldn't sell Matuidi or Khedira, then spent 40M on a 19 yo winger who so far hasn't lived up to his potential.
Szczesny
Cuads De Ligt Bonucci AS
Ramsey Pjanic Rabiot
Dybala Higuain Ronaldo
Buffon
Danilo Demiral Rugani Chiellini MDS
Khedira Bentancur Matuidi
Berna DC
Which leads us to the last summer, we terminated contracts of the expensive old guys we failed to sell in previous mercatos, Higuain and Matuidi (Sami in January), we raised plusvalenza on our best mid just to overpay for another mid, younger worse version of himself, got a unproven kid from Schalke who could end up costing us up to 30M. Chiesa was a great deal even at 60M and Morata was a good panic buy after we failed to land Dzeko or Suarez (but could potentially cost us 50M as a 30yo with a sickness, if our option is a gentlemans obligation). And most importantly we experimented with coaches along the way.
tl;dr we downgraded ourselves annually, and we spent big money on the way of doing it. How we did it? By looking at random market opportunities and fishing for plusvalenza instead of focusing on building/retaining a functional team once we landed Ronaldo. The guy has nothing to do with it, it's the fools making those random deals that are at fault