I didn’t have that take. My understanding is the only time you’re guaranteed to have low-cost ticket availability is when the flight is initially allowing booking.
Later, as more tickets are sold they have more information about the flight and thus they may make adjustments to the ticket pricing. Either opening up more low-cost tickets that were sold at the beginning of the flight or reducing the number of low-cost tickets to meet high demand.
It's worth pointing out that the initial fares will be comparatively cheap fare classes, but they may not be the cheapest prices depending on what happens with demand closer to the flight date. You can often get better fares by waiting if you're flexible.
Later, as more tickets are sold they have more information about the flight and thus they may make adjustments to the ticket pricing. Either opening up more low-cost tickets that were sold at the beginning of the flight or reducing the number of low-cost tickets to meet high demand.