No offence dude, but that's just plain ridiculous. I think even the six year old girls would stop watching if they did something as stupid as that.
For starters, the Mane Five aren't really cut out to be princesses. Pinkie Pie or Fluttershy as a princess? Must I even justify? Applejack could be okay, but she wouldn't want to be one anyway. It's just... What?
Also, the writers and animators you seemed to make out as mean or something. Well, they're clearly not. And they're not stupid. They know it would be a ridiculous idea. They have no reason to be princesses, thus they shouldn't be princesses. It's just common sense.
What I would be concerned about is the rainbow spool of thread having a connection to the Rainbow Power ponies. But anyway, I digress.
It's just too stupid an idea for it to be carried out, even by Hasbro. And that is saying something.