If they don't sell now, they'll wait for the midtable irrelevancy stage of decay("Glazer Apocalypse"). Brand will lose a significant amount of it's value and they will probably regret not selling (much)earlier for relatively bigger amount of money, considering future inflation. They're not smart businessman, they're just rich, spoiled, incompetent, grotesque goblins who inherited the wealth(assets) from their father, Malcolm Glazer, who originally bought the club in 2005.
I don't think they'll wait for the relegation battle phase of the Glazer Apocalypse to cash out and sell. Value of the club/brand would tank completely then. But even that is not impossible with them, just think it's unlikely.
