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]
Im not so sure Moyes would have lasted getting top 4.
Your basing everything on the assumption that everything said in public was the utmost truth. United on the inside might very well have wanted many others before Moyes, and may have seen Moyes as a buffer until a big name became available. They aren't going to let that be known though are they, United aren't a club to come out and say this is our interim manager, we have no confidence in him long term but until we get our guy he'll do. No, they come out and say we believe in this guy, then if by some minor miracle he goes and wins the league, or remoulds the team into something that has a future, bonus, if not, get a proper manager. As it was, he was crap in all areas.
If Moyes had got top 4 last year he might have stayed, but personally I don't think he definitely would have, im not sure top 4 would have been enough necessarily as his management in general was piss poor. The team looked shoddy, unenthused, tactically inept and like it was going nowhere. He had no ideas and was being tactically outdone at every turn. No chance can I see United looking at his first year, even with top four and thinking "yes, this is the guy were going to give 2-400million to over the next few years to rebuilt this team. Instead id be thinking, "thank feck he managed to keep top four, now lets get a proper manager in to do the rebuilding".
As for the stuff on Fergie, sounds like nonsense to me. Fergie spent 2 years before he retired openly meeting with some of the best managers in the world who coincidentally would potentially be available. And unless you have access to Fergie's diary, there no way you can say for sure that Moyes was Fergie's first choice. Much more likely was he met with Mourinho/Pep etc over his last few years sounding them out. When it came to it the main targets were not available and so Moyes was brought in as a solid place-holder with potential to be better.