It isn't an ultra short term view, it's just back into the real world for United where the managerial position is actually a big decision to make each year. It's unrealistic to assume a manager will simply just last 3 years, and even more unrealistic the further you look ahead. Alan Pardew is currently the second longest serving manager in the PL behind Wenger and he's only been at the club 3 years and 136 days (only 8 of the current 92 managers in the football league have served over 3 years).
You have a very tough year ahead to get back into the CL - many on here think the current squad needs investment, and with investment comes risks. Even if they do slot in nicely, one of Liverpool, Chelsea, Arsenal, City and United have to miss out. Would you bet against any side in particular missing out at this stage in place of you? (I'd bet City and Chelsea don't drop out, so in my mind it leaves 2 from 3, but that's a different discussion). If you miss out on CL, do you give him more time than you gave Moyes? All hypothetical of course but quick manager turnaround is part and parcel of football these days.