Conte gives us two full seasons and we're in way better shape than anything Nuno can do
This is precisely it. People claiming "Conte will only leave us after 18 months" seem to be labouring under the notion that:
1. Everton wouldn't sack a manager who has underperformed after 18 months
2. Any manager who "we could keep" is clearly not good enough to interest the clubs we are trying to join at the top table, so why would he be good enough for us and why would we want to keep him?
If a manager hasn't improved us after two full seasons, he is not going to. The game has changed. The average tenure is 18 months. The key is not to "keep the manager", it is to have the financial means to get rid of him if required or replace a poached manager with somebody of similar ilk. In that sense, lose Carlo, get Conte makes perfect sense. Lose Carlo, get Nuno means we had no real plan for Carlo other than bragging about getting a kiss from the loveliest girl on the dancefloor.