Depending on the price difference, before COVID I would commonly book a lower fare with a suboptimal-but-acceptable time of day and hope for SDC. These were typically 1300+ miles segments and I was about 80% successful.

However, for the last few years there have been fewer available F seats even 2-3 days out, to say nothing about the SDC window, often on a high percentage of the available flights for the day. I came to conclusion that I either had to use a guaranteed RUC or pay for F. I recall seeing similar experiences reported a while back.

It's hard to know but it's posslble Delta's data predict a net increase in revenue from the interaction of 1) forcing WFBF on higher-priced flights people want, 2) lost annual revenue generated/lost by flyers who even occasionally use SDC, and 3) potential revenue "lost" by buy-early SDC-late.