Just to recap: Manchester United recorded the fourth-highest revenues of any soccer club in the world, per Deloitte's most recent accounting. They pay the fourth-highest wages of any soccer club in the world, per the estimates from FBref. And they finished 15th ... in their own domestic league.
Of course, these things happen sometimes. Jürgen Klopp's Borussia Dortmund finished seventh in 2014-15. That same season, José Mourinho's Chelsea won the Premier League, but the year after that? Mourinho was fired during the season, and the team finished 10th.