I'm a bit surprised the dragon was added without the ability to add an achievement for killing the dragon. If an achievement for it is added in the future I can imagine people not being happy that they'd have to restart a world just to get the achievement. I feel like this is something 4JStudios should've considered and they should've added some sort of DLC (as reading above that seems to be the only to add achievements) to add more achievements or if there was any other way to add achievements then do it that way.
Either way, people would have had to have paid to get the achievements. Microsoft has rules and 4J must follow them.
I highly doubt people, even achievement whores, would have paid for The End just to get a new achievement.