When writing the “history” of my NationStates Equestria, I used the same headcanon, although instead of Tirek, I figured it was Discord (during the period when he misruled Equestria) who obliterated the ancient Unicornian royal line. That explains why there are ponies like Prince Blueblood running around–there were many surviving descendants of the old Unicornian royal family, but no one could say for sure who was first in the line of succession. Celestia and Luna were picked for their heroism in defeating Discord, but because it couldn’t be proven that they were first, they took the throne as Princesses instead of Queens.

@Background Pony #BF6B  
This is exactly the version I read. Pity: girls should always look for larger horizons. I’d rather be a king than a prince, but the prince/princess figure is sooooo stuck in our heads thanks to many fairy tales…

The in-universe reason why Celestia and Luna aren’t queens is because their goddess had her creativity castrated.

I like this idea and respect your creativity but I can’t stop laughing at how it looks like Luna is rolling her eyes at the explanation.

@Background Pony #BF6B  
Good thing that Dracco didn’t mind queens in their magical equine brand three years beforehand.

@Background Pony #28FC  
Funny thing is, I guess Faust-senpai wanted to call them Queens but ran into trouble from Hasbro. XD

Well you’re wrong, but then again it’s headcanon so…