They do sell out upgraded seats every flight.

In the later 90s, I used to try to upgraded at check in. It was usually a 2x cost of the was a seat or less vs 3x of you got first class ahead of time. Since the 00s is pretty much never an option. Most flights are over sold and getting a decent seat (prefer front row) is really hard.

And the past couple years, even then they're often sold out when buying tickets less than months ahead of time. Mostly have traveled in occasion for work and mostly shorter notice.