Manchester United had an interesting 2016-17 season. They finished 6th in the Premier League under the pragmatic Jose Mourinho after spending over 100 million dollars in the transfer window to upgrade their squad. Despite the 6th place finish, the Red Devils won the League Cup & Europa League against an Ajax "under 23" squad to earn a backdoor entry into next year's Champions League. So the question is, was this season a success? This question depends on what standards you are using.
In terms of a grand historical Man United perspective this past season has been horrible. Manchester United is the biggest and most successful club in England and they are always expected to compete for the Premier League title. This season the club finished 24 points behind the champions Chelsea, while the club was playing "boring" football. This is not good enough for a club that prides itself on it's past success & attacking prowess, which was hard to come by this season. If you look at Man United in terms of an historical perspective, this season has been dismal to say the least.
However; this season can be understood as a success if you look at it from a rebuilding perspective. Since Sir Alex Ferguson retired from the club five years ago, the club has been rebuilding its squad and has lacked the leadership and organization under Moyes and Van Gaal that led to twenty years of glory under Ferguson.
Since 2013, the club has only been in the Champions League group stage once and that season they crashed out in the group stage. So, winning a continental trophy (even if it's not the Champions League) and earning a Champions League birth can be considered a success for a club looking to reignite its fan base and global brand. So, in a sense this season can be looked at as a success as Manchester United takes a step toward becoming the European behemoth once again.