But it started with Ronaldo investment that led to Marotta dismissal
To be honest it probably started before that when we sold Vidal and Pogba without properly replacing them. But I think we had a good enough squad remaining to still be succesful, which we were with Allegri. We should have been able to manager way through it and were doing that up to a point.
What has really cost us has been overpaying for average but expensive players (either high wages or fees), whilst also losing important players with grinta (Lichsteiner, Mandzukic, even Tevez):
Rabiot
Ramsey
Morata
Kean
Kulusevski
Bernardeschi
Arthur
Higuain (Not an average player by any means. He was an excellent striker and I really liked him but 90m to release a few seasons later was terrible business)
Bonucci (Huge wages, we brought him back on higher wages after flopping at Milan than he was on at his peak before that)
Just about every transfer decision we have made over the past few seasons has been a poor one. But I struggle to think of worse deals than:
Arthur / Pjanic
Spinazzola/Pellegrini
Romero
All absolutely woeful transfers.
With hindsight the Ronaldo deal didn't work out but I can see why we gambled on it.