Ballad of the Crystal Ponies. For months we have been listening to this awesome earworm, but did we ever stop to consider what it means? The very beginning outlines the situation, and it ain't pretty.
Princess Cadence needs our help.
Her magic will not last forever.
I think we can do it,
but we have to work together.
Oh, we've got to get this right.
Oh, we've got to make them see.
We can save the crystal ponies
With their history.
From here on starts the
Wild Mass Guessing.
Princess Cadence will loose her magic, or die. Seeing how alicorns can live for millennia, it's going to happen soon. Her magic sustains the crystal ponies, who need magic badly (failing that knowledge will do?), or she is in fact the last of them. She is married to a unicorn, and not an alicorn, which means it's an inter-species marriage and might not produce children (or children who can reproduce). The lack of alicorn stallions might actually be a bigger problem than anyone realizes.
The memory and knowledge of the crystal ponies is dying out. Maybe they were a separate species that no longer exist (either dying out or intermingling with the current population) or a long-fallen kingdom. Cadence wants the main chars to preserve this knowledge, thus saving them. Or failing that, their memory.
Or it can just be Twilight exaggerating what is simply a renaissance fair.
Und wenn du lange in einen Abgrund blickst, blickt der Abgrund auch in dich hinein.