I don't know what happened to MU, they haven't been able to get out of the inconsistent zone. The team development carried out by several managers after Sir Alex Ferguson, could not bring glory to that era. It's difficult to find the right replacement of Sir Alex Ferguson. Arsenal is lucky after Arsen Wenger, currently there is Arteta who seems to be able to make Arsenal grow even though he experienced a difficult phase which made Arteta receive a lot of criticism from fans.
Manchester United glory have been over since leaving by Sir Alex Ferguson and several manager failed for winning domestic league until champion league trophies. Have been 12 years ago since first time David Moyes until right now Erik ten Hag become the manager still not any one of them success make Manchester United finish on the top standings position or becoming the competitor for Manchester City for winning Premier League trophy.
Difference with Arsenal, they had success with Mikel Arteta become the manager for replacing Arsene Wenger position, indeed still not winning any trophy yet but Arsenal keep solid performance and always finish on the top fourth standings exactly last season almost close become the winner of premier league.