Conte is a top manager, that was never in doubt.
It went wrong here for the same reason it always goes wrong, Levy didn't match his ambition in the market. Of course there were many unfortunate circumstances during the 2nd Conte season like the mid-season WC, several friends dying, his illness etc, but you cannot hire a manager like Conte and give him Lenglet on loan when he asked for Bastoni. That kind of nonsense lit the fuse and it was only a matter of time before he exploded. He asked for a top player and was given a donkey.
To succeed with Conte you need complete commitment from the entire club, otherwise he will go off like a nuclear bomb. If the chairman wasn't willing to give that commitment, no point hiring him. I think Levy thought signing Perisic on a free and a big salary would be enough to placate him, which was absurd. We needed so much more.
It would also say a lot about how little most fans understand football. The reputations and price tags on the shirt aren't the most important thing, it's the qualities of the players and the overall team cohesion that matter.