Faulting the manager is crazyy, yes he is also taking responsibility but do you want to blame him for the individual error caused by Casemiro during the game against Liverpool? Do you want to blame Erik Ten Hag for the players not converting the chances they had against Crystal Palace? What about the individual error caused by Harry Maguire during the draw against Brighton? How about the individual error again from Harry Maguire and Christen Eriksen against Twente? Y'all Manchester United are bloody inpatient punks who wants to see quick change that won't bloody last. I'm disappointed, and even if they want to sack EtH, y'all want to settle for mediocre managers. 
True. The decline of Man United that is happening now, it feels unfair if we only blame the Manager, because as you said individual mistakes are also the main factor that makes Man United like it is now.But in essence the decline of the team, of course the one who must be responsible is the coach. I see Erik Ten failing or not fully controlling the dressing room. Either because his toxic players are too strong or Erik Ten is weak. If you want to be firm, be firm with all players without discrimination. Because whoever the coach is, if the players have high egos and are difficult to subdue by the coach, then Man United with its new coach will remain like this. So in my opinion, Erik Ten must be brave with his players and be given more power by management to park players who rebel against his plans/tactics.
Because if we look at Man United now, it's very strange, the players who come in are generally good at the club initially but when they enter Man United they flop, and vice versa. There is something wrong with Man United, is it the manager or the management? But in my opinion both of them should introspect.