I think These Events (The Return of Nightmare moon, Discord and Trick, Transforming Twillight to Alicorn and more) all part of big plan by Princess Celestia.
You see Princess Luna and Princess Celestia born in age of Alicorns was the normal kind of ponies, alicorns is a state where a pony understand the ultimate harmony AKA Friendship.
You See ponies back then turn into alicorns when the understand the magic of friendship, like now days ponies get they cutie mark.
But with time go by less and less ponies turn into alicorns tell not that so many lift.
The last two alicorns treated as royalty (Princess Celestia mother and father), they had two daughters Luna and Celestia, and one day they die and let the two filly alone.
and a lot of things happened (Discord, sambura and nightmare moon ..... )
But Princess Celestia still remembers the tales her mom tell her about the Alicorns and want to return these days.
If she can turn a pony into alicorn, just one she maybe can make the ponies believe that they can do it to.