For the Glazer's it would be like cutting the head of the golden goose. In short, they have no need or reason to sell. With the popularity of football Utd are going to keep making huge amounts of cash every year almost as a given. I can't see the Glazers getting rid of such a consistent cash cow unless an insane offer is made. Currently they can just sit on their arse with next to no risk and keep taking a tidy sum each year. Why risk that for anything but insane money.
They would need to research another club and/or sport, get a new team of suits together and possibly invest cold hard cash, something they do not like to do. That is a lot of effort and indeed, risk for people who don't have to do that when they can continue to just watch the cash role in each year.