maybe it's because there were no plans to get rid of the 733s. Remember, they are leaving the fleet now because DL is able to reject the leases in bankruptcy. Shoots the heck out of the theory that DL has planned a bankruptcy all along (which some here have asserted).
I will grant you that the whole airline within an airline plan has been very mismanaged at Delta. Even though they face a very different competitive environment than DL, it is on the subject of airlines within airlines that I give credit to AA for assertively saying that creating an airline within an airline that fixes only part of the parent airline's problems makes no sense.
The only valid reason DL had for creating Song was in order to create a higher value product to compete with certain carriers - but that doesn't make a whole lot of sense to do when the discount carrier ends up with a better product in lower yielding markets than traditional more business oriented markets. I can wish Grinstein would have gone with his gut instinct earlier and never let Song get off the ground - and I bet he does too.