Typically, these seats open up 24 hours ahead of time if someone with special accommodations has not already booked them. And I completely understand that NRSA travel is a complete crapshoot. But economy comfort seating is in place for the more loyal passengers (elites) or people who pay for it. It is a perk much like getting an upgrade. Delta has spent a considerable amount of money trying to make the overall travel experience better for those of us who spend a majority of our time in airports, hotels and planes. I would think they would want to continue that on each and every flight.
This was an oversold flight anyway. 26 on the upgrade list after clearing, 36 on standby.