Despite the empty seats, the flight could be in an oversold position but they are still willing to sell an econo seat. Depending on route and time of year, we know of overbooking by 120 and even more--not all those people have seat assignments and the airlines hope (and know correctly most of the time) that most will not be flying that day.

You stand a better chance of op upgrade than if you saw lots of low fare class seats available for sale still. It depends on status, timing, charm . . .