Do not think it is dark magic, it is about corrupt magic that manifests itself through the envy and displeasure of the earth ponies. The hole in the trailer definitely looks like a portal to a second dimension, magical glimpses could be seen and there are many indications that Sunny had plunged down there, from where a rainbow - pony magic - came out. She and many others had not thought about what magic for the earth ponies really is, and may have thought the internal magic is essential for the earth ponies.
We know from G4 that the earth pony magic is much more than just internal magic, as we saw with the Pie siblings - but it was not as much vented as with the pegasi magic and unicorn magic. Here is a potential to learn more about the earth pony magic. The insurance companies in Maretime Bay are in for a bad time ahead.
“It’s like she’s forcing utopia on them.” Hard to say, but it’s clear that the magic in the G5 is not stable, and it must be emphasized that the earth ponies HAD NOT GOT MAGIC when the movie ended. Then it meant that the Unity Crystals have built-in errors. And it is also possible that something may be wrong with the active magic that may have led to the loss of magic in the forefront of the G5, while we know that there is almost nothing in the middle of Equestria from the G4 where Ponyville was located.
Maybe the non-ponies fled when Equestria collapsed because the magic had become unstable. And that may explain the chimera animals that can be seen in G5, they may have been exposed to uncontrolled magic that Discord in the IDW comics might prevent. We know he took the Unity Crystals with him to the ruins of Canterlot.