As upgrade space has disappeared on domestic flights, and I got tired of dying at #1 on the upgrade list, I have been buying F. It is usually a few hundred extra, and worth it to me. Especially as I’m no longer 1K. You are treated better, have more space and sometimes decent food. I suspect there are a lot of people like me - used to have status, now pay for F.

i also watch the price - if it seems high at first, I may buy economy, monitor, and snag a price drop in F or cheap cash upgrade. Those still happen.