In terms of acheivement, what did Liverpool gain by finishing 4th that United did not also gain by winning the Europa?
You are right, though, there was a time when United - along with many other clubs I might add - did not consider the Europa League to be a worthwhile venture. But that was then and this is now. The Europa became significantly more prestigeous after Uefa, aware of the fact that their trophy was considered to be a hindrance to those who entered, saw fit to grant the winning team a spot in the CL the following season. With that added incentive, the Europa league is now very much worth winning.
2nd, 3rd, 4th, winning the Europa. One and the same thing, but with the Europa you get to bring home a trophy. Are you really telling us that you wouldn't swap places in an instant?