I would argue that most more savvy flyers here would end up losing under such a system. Surely for every person here who knows to call in/request an upgrade at the right window, there are ten more clueless frequent flyers who leave it to their secretary or TA, or only request at the gate, or are unaware of the window.

Automatic upgrades would go a long way toward levelling the field which is to the disadvantage of the more knowledgeable/more proactive among us.