See, I think the opposite. It's much better to take over from them in the summer than it was after Fergie left. Fergie didn't even finish outside the top 3 for about 20 years straight. Taking over for next season, whoever it is, the fans and board would be happy with a top 4 finish with some attacking football. They'd have to do more in subsequent seasons, but initially, that would be the bar and that is very achievable for a good coach like Poch -- especially if they were to buy £200m worth of players.
This is the perfect time to take over what is one of the world's biggest clubs, because Moyes/LvG/Mourinho have lowered the bar so much as to what constitutes a good season, at least initially. You will never get that at Barca/Madrid/Bayern. Relative to those clubs, a coach will get some time to get it right at Utd.