The last time the Dolphins won? Far too long ago. And totally irrelevant to the discussion at hand. I'm sorry, but just because my favorite team hasn't won a championship in some time does not render moot whatever points I may be making in this thread. If winning a recent championship is a requirement for opining, then only Colts, Steelers, and Patriots fans should be allowed to talk.
Point is, Jerry Jones is handicapping the ability of the head coach to do his job. And Cowboys fans can't see it, because you still think it's the early 1990s.
You asked about the success of the team. Success in the NFL is defined by two things:
1) Revenue
2) Super Bowl Championships
Since Jerry has been in charge of the Cowboys, only two teams have won as many Super Bowls as the Dallas Cowboys (1993,1994, and 1996), they are San Francisco (1989,1990, and 1995) and New England (2002,2004, and 2005). I am being generous to San Francisco since Jones technically purchased the team after Super Bowl XXIII.
Dallas is 163-144 (including the playoffs); 3 Super Bowls with 12 playoff wins. Out of 18 seasons, they have had 10 winning seasons.
Relative to other teams, that appears to be successful.