Noone outside of the company will ever know, yet I see this as a revenue-positive situation. Except for some very specific routes (which the A319 may not end up on), I don't see this being a revenue negativity issue.

As I've burned through mileage with premium seats (domestic and international) up to 11 months out and was able to assign seats, I've noticed a pattern that for a majority of the time, I'm the only person with a seat selected (maybe 1 other), and the seats don't start filling up until 1-2 weeks out (clearing buy-ups, op-ups, upgrades, etc?). And if upgrades for the last batch are held back for gate-upgrades, to leave room for last-minute premium seat buyers, DL still gets the premium fare, and the GAs will just gate-upgrade the empty seats.

I suspect that these smaller F-cabins will fit in well with medium-distance routes that have heavy Y-purchase, and light Elite pax (DTW-PBI/FLL/MIA?). In the short-term, figure it's better to get 6 more paid-Y seats, and loose 4 upgrade-F seats (that offer no immediate cash-flow).