Lunacorva said:
Except you're failing to take into account that The Dazzlings became powerful in the finale by absorbing the Equestrian Magic that Twilight brought into the human world. Saying that they don't scale because they were weak at the start of the series would be like saying that Tirek doesn't scale because HE was weak at the start of the series, or that Cell can't be Solar System level because he lost to Piccolo in his first fight. The whole point was that they acquired enough power to be able to fight the Rainbow Power head on.
Tirek absorbs almost all the normal magic in Equestria + that of Discord.
Cell absorbs two Large Star level characters and then increases his power again, after that.
The Dazzlings feed on excess magic brought over by Twilight and Sunset. That is not remotely the same as if they had fed from the source, or if they had been stated to do something such as drain all of Alicorn Twilight's magic.