Have never fully understood the “availability” of fares you can’t actually purchase anymore. It sounds fishy to the customer. Airlines would be more transparent having all/most of their remaining seats available for SDC with a reasonable reserve for paid walkups. Yes, there is an argument against that with people buying possibly-cheaper, later-in-the-day tickets and bringing them “forward” to game the system.

But a reasonable ability to SDC for free allows frequent flyers to actually… fly… frequently.