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.

Urgency is one of the key motivators for using dark patterns. Wouldn't be surprised if Delta defaults to "1 seat available at this price" for all busy flights within X take off time regardless of the availability.