Ferguson is not the last good coach for Man U... It's just that the club management hasn't been able to find a really good coach yet... Man U have really good players and superstars, but without quality club management they will never get good results... The situation reminds me of Barcelona when they played horribly, but after Xavi came in, they were able to finish second in the league... About the same decision is needed for Man U...
Basically most of the coaches after Sir Alex are good coach but perhaps the expectation from fans was too high because most people expected to see similar achievement as what Sir Alex had. In fact Sir Alex needed several years before he could build the team into what we know as the dream team. After Sir Alex, most coaches were sacked out quickly. I'm not sure what will happen to Erik Ten Hag, I heard that he will rebuild the team by changing many players. I guess Erik Ten Had will be sacked as well if the expectation is still too high.