FDJ, the roll of the dice I was thinking of was a separate operating division and separate brand built almost solely to inflict pain on JetBlue. But as you say, there is no longer a reason to keep Song separate. Why? The brand didn't serve Delta all that well. More exactly, Song became well known in certain circles as a good product, but it only made Delta coach look bad by comparison, not JetBlue. Some passengers preferred Song over JetBlue, and vice versa, but by most accounts the products were pretty comparable, and markedly superior to most domestic coach.
Getting rid of Song is a net positive for Delta, and will serve to improve Delta as a brand, now that they're not competing aganst themselves anymore. But all the costs associated with building Song as an individual brand are now lost. Furthermore, unless the product improvements are distributed to all domestic coach, which would be very expensive, it will create brand confusion among Delta customers. It'll be a crapshoot: TV's or no TV's? Depends on the route, the equipment, maybe even the day or time slot. Nothing you can really count on, so it will be difficult to market the improvements as a service commitment. The best you can say is that on a given day, you'll probably get an airplane with TV. At least with Song, you were buying a known product. Now the product is less known.
Sure, there's more to Song than the TV's, but they do create buzz and a service that sets itself apart from other legacy coach. Now your customers won't know what to think. A marketing challenge for sure, but still I think a net positive for Delta. And the rest of us.