Singing was a little cheesy, though :p

It seems that the mane 6, headed by twilight, are destined to replace Luna and Celestia. I thought that to myself once we see that Luna and Celestia cant use the elements of harmony anymore, but now it definitely seems canon.

Also, Seeing twilight have to look into other things, IE learning to project pure evil, makes that seem more likely. A Leader of a magical world must understand most the light and the dark, to ensure the light prevails. As we see, Celestia, the gracious ruler of equestria, can also command pure evil, as nescessary.

Additionally, this is the first time they actually KILLED their enemy, rather than banishing them for 1K years like usual.