There's a phrase used in business -
snout in the trough. When I've heard it said of people, it meant you work for years to get to the top. Once you're at the top you take advantage by manipulating pay and conditions. Getting other people to award you bonuses, pensions, and pay-rises. You can't get these benefits unless you're at the top.
People at the top, don't concede authority because shared authority makes one less important for the company. With most public companies the shareholders will stop a bad or weak CEO by sacking. But Glazers only see the bottom line - profits and good accounts. It may take a few years of sporting mediocrity for profits to be really hurt. So strap yourself in. Get ready for the long haul. They will not sack him, nor force his to share control over United's playing staff.