That argument wouldn't work, though.
If Moyes got top 4 last year, he'd have stayed.
So if he stays and we keep getting top 4s, they have a problem on their hands with a manager who is doing "ok".
If a top name manager became available last Summer, for example and Moyes got top 4, there's no way that they would fire Moyes to replace him with a top manager.
Moyes was genuinely believed to be the chosen one. They wanted him to stay for the long term. They wanted him to win everything.
Notice that when Moyes was fired, Fergie was left completely out of the loop. And when LVG was hired, Fergie was once again out of the loop.
They realised Fergie was a great manager, but was terrible at searching for a successor - I think they realised this.
[they = the powers that be, who decide who gets the job]