Your post got me thinking. The lower sub 24 hours fare could be an attempt to increase the number of customers who buy up to change their flight time on the day of travel.
As you said, customers are unlikely to buy up from $49 to $200, which was the choice I faced for many years. Buying up from $39 to $108 is much easier to accept. That was exactly my situation last week and I paid it (due to having a Companion with me). Chalk up one win for this fare experiment.
Another factor is that people who buy a non-BS ticket inside T-24 lose any boarding priority they had. A-listers can board before B1, but everyone else would need to pay $40 or whatever to board early.
That said, when I've seen these lower fares appear, BS is also discounted approximately equaling what the WGA fare was hours earlier. If you felt the need to board early a last-minute BS purchase with its higher points earning might be the deal for you.