I think Sony was *clever* (if that's the the closest word that comes to my mind to describe it) for not releasing This is it as a single, because they probably figured out that when we heard it and realised that it's just a very old song, a demo, not finished with the brothers adding their voices over it after MJ's passing... the fans would be angry and decide to not buy the single to show their anger and just wait for the album..
Does that make sense?
I would have loved a NEW song that we could have put #1 so Michael would have had 40 years of #1s...