if you honestly believe that, then you only have to look at manu post sir alex, and inter post mourinho to see what a difference a coach can make in the team
Mou left at the perfect time, the ship was sinking and major players were at the end of their career
United - Granted. SAF held a mediocre team together. But of course, Moyes was ridiculously stupid, I won't even bother going into his amazingly retarded decisions. But you have to remember, this is in the BPL. For competition, United have 2 free spending Oil clubs, Arsenal and a bunch of Europa league positioned clubs that have bigger spending power than most of Serie A teams do.
We, on the other hand have a vastly superior squad to all the other teams, a better budget (If somebody like Napoli doesn't sell key players obviously) and some genuine world class players. So unless we hire a manager who almost goes out of his way to make stupid decisions, we won't vanish when Conte leaves. Not at all.