"There was money, but we couldn't sign the player we wanted" was just the recurring theme of every window though. It talked to the same problem that we couldn't land anyone and just seemed way too convenient especially when we had 2 years of record profits. It amounts to the same thing.
Poch was very vocal in his early days about not being able to settle his squad for pre-season and get off to a fast start. He called the club out for the late, late shows and the procrastinated transfer cycles. He was hitting his head against a brick wall.
As for Tanguy and Gio, it is not Poch's job to do the physical and mental assessments of these players. That due diligence process belongs with the recruitment team. It is also fair to say that they were way more likely to thrive in a Poch system, than the hoofball Jose played or the 3-4-3 counter that Conte plays. They became a wrong fit as soon as Poch left the building. Tanguy will just waste his entire career anyway. That kid is mentally flawed.
There is a sort of irony that the managers gets the blame for some of the acquisitions, but then some of the players get blamed where it is clearly the manager's disposition to treat them awfully. As you say, there is no single root cause, just some from column A and some from column B. I personally find it hard to blame Poch too much for. the acquisitions as he was operating with negligible net spending until it was too late. Hence why I started this article at 2017.
I'm so glad Levy has stepped away from football ops. There's your biggest root cause.