I think this all boils down to what you define as successful.
You are clearly are dismissing the careers of Bruce, Hughes, OGS to name the three obvious ones. You've clearly dismissed Keane's stint at Sunderland, bringing them from almost bottom of the championship to top and promotion. And even the achievements (although less impressive than any of these guys) of Robson.
But of course that's because success by your standard are 'Everton class clubs'
. That's what I'm laughing at. And before you say it, no none of those managers achievements qualify them for the United job. But to dismiss them as unsuccessful managers, three of whom are current PL managers, is disingenuous to say the least.