My first reaction to Delta to Eliminate Discount Carrier Song was, "of course it failed." It failed because they didn't burn their bridges, didn't really commit, didn't do anything but a pale imitation of Jet Blue.
But then I realized that Song wasn't a failure on at least one level... it allowed a stodgy brown company to move fairly quickly and to discover the power of story telling. Everything from the organic food to the paint job was about telling passengers a different story. Song had trouble keeping that promise, but at least they tried.
So maybe Delta learned a lesson about flexibility and speed and risk. Failure is rarely fatal.