Seems to me the flaw in this plan is that people are cheap -- you'd encourage a lot of them to sit out for the drop, drop, drop in prices in the last 12 hours. So you'd miss out on:
A) the logistical sanity of just having the plane mostly full well in advance.
B) A good chunk of the interest you make from holding on to several hundred dollars per passenger for what is often a few months.
plus, you'd probably end up with a lot of grumpy people who missed out during the last minute firesale. Sure, its their own fault, but people don't care -- you spill your fries, it's McDonald's fault, you show up late on Black Friday, it's Best Buy's fault that they only had 8 doorbuster sale HDTVs. People who missed out on a seat due to their own lack of planning would probably just rage at the airline.