the most successful clubs have long term managers because the managers are successful, which is fairly fucking obvious really.
Manchester United;' success under Fergie:
* Premier League (11): 1992–93, 1993–94, 1995–96, 1996–97, 1998–99, 1999–2000, 2000–01, 2002–03, 2006–07, 2007–08, 2008–09
* FA Cup (5): 1989–90, 1993–94, 1995–96, 1998–99, 2003–04
* League Cup (4): 1991–92, 2005–06, 2008–09, 2009–10
* FA Charity/Community Shield (9): 1990*, 1993, 1994, 1996, 1997, 2003, 2007, 2008, 2010 (* shared)
* UEFA Champions League (2): 1998–99, 2007–08
* UEFA Cup Winners' Cup (1): 1990–91
* UEFA Super Cup (1): 1991
* Intercontinental Cup (1): 1999
* FIFA Club World Cup (1): 2008
He started the job in 1986.