That's not really so.
If everyone went for the lowest fare there would be no premium class passengers. That clearly does not happen.
Spare aircraft in the old days is a bit of a myth as well, especially at outstations away from base. Goodness, carriers like Dan-Air (relevant to BA because they eventually bought them out) used to operate from up to a dozen different UK departure airports with just one or two aircraft at most - and no spares, running secondhand aircraft 24 hours a day in season.