It is very difficult for Manchester United to get into the top 4 places. Considering the tough rivals in the Premier League along with the point difference, I think it would be better for the league to fight to get to the European league. They had some good games against the best teams in the league, but considering the poor performance in the UCL this year, it was not the year they wanted at all.
A month ago I will have second your point that is Manchester United cant challenge for the top four with likes of Tottenham and Aston Villa, but right now I will tell you Dey can actually go into next season playing champions league football. The two reason for me is, both Aston Villa and Tottenham have been dropping points lately which reduces the gap to just 8 point before Aston Villa in the fourth place. If you check the upcoming or let me say the matches left you will see that this two clubs have tough fixtures coming up, although Manchester United also have some fixtures coming but with their injured players coming back the momentum or spirit is currently very high and they can take on just any club they wish to.
Also the fifth position in the league can also be that of a champions league spot because of the increase number of participants In the next edition, with the premier league looking to have a slot and United can still get luck with that.