But there's the rub: nobody can know there will be available seats. So your "if people know.." is false. Besides, CO is already selling last minute seats on the cheap. Look at their weekly specials. So at least on some level my scenario is in effect already.
Weekly Specials are not a good example. Too restrictive. Must fly out on Sat and return on Mon or Tues.
One of the problems with your scenario is this... a meeting just came up where I have to travel in two days. I can look at a seat map to get an idea of how full the plane is... or use one of the other services. If I know tickets will be discounted in a few hours, I will wait to save 50% or so on the ticket. Is it a lock that a ticket will be available? No, but I have a pretty good idea that one will. All this would do is train last minute travelers to become a little bit more last minute in hoping to score cheaper travel. You would be taking away a major revenue source from the airline.
That's why they don't care about the specials... business travelers don't fly on Saturday typically.