I'm not sure anyone would prefer coach to first class ...
You have to realize not many people get first class tickets. Most people in first class are that airline's elite level frequent flyers (silver/gold/platinum or whatever it's called on that particular airline). These persons can usually only upgrade a certain amount of time before the flight. For instance, on Delta, Platinums can upgrade at any time before the flight (using the "standard" upgrades), Golds at 72 hours, and Silvers at 24 hours (this is using the "standard" upgrades that you'd use on such flights. There are upgrades that can be used at anytime but they are less prevalent). Even though First looks pretty empty now, you can be pretty sure these folks will fill up the first class cabin.
If you wanted to upgrade by paying, it would be very expensive ... 100's of dollars. Less expensive upgrades are only available to the above mentioned elite level frequent flyers (on Delta, other airline's rules vary). Regardless, First is almost always filled on Delta with such elite level flyers anyway. In my four years as a Gold Medallion with Delta, I can count on one hand the times I did not see First Class full by the time the plane left.