In my experience this replicates routinely for Delta Main Cabin fares for non-elites on leisure/non-corporate travel once only “Preferred Seats” are left in the economy cabin.

What I think the other carriers do well is detect when there aren’t non-charged seats available for advance selection, and remind you that you can continue without paying for a specific seat and still be guaranteed a seat on day of departure.

Delta hand out “free preferred seating” as a perk to all sorts of categories of traveler though. I wonder if that complicates the prospect of offering that kind of warning.