The Glaziers know there is at least a fair chance of some sort of deal coming down the line, be it a shift in the TV revenue models, or a super league, or just evolution of modern football comps, but the chances are at least decent that something big happens, especially now Saudi are throwing crazy money around as well.
Whenever I think of people like Glaziers, I think of the following mantra "they don't want *some* of the money, they want *all* of the money. They probably feel a 100% deal right now prevents them from getting an even larger slice later on. Should the TV deals go astronomical combined with some league structual reorganisation, suddenly 8-10b on United isn't so fanciful as it currently seems, in which case they can get their 1.5b now, and perhaps get another 6 down the line, meaning they've made 1.5-2b on Jassims offer whilst being no worse off financially in the meantime.