I don't think Celestia could be not vicious enough to fail to use dark arts of persuasion, but still be vicious enough to threaten a thousand years of solitude.

I agree that violence is not appropriate to the genre, but the threat is also inappropriate. Perhaps explicitly communicating 'we respect you, but we think that you are mistaken on this one issue' would help make Celestia seem like the Bad Guy in the end, for demanding banishment and other unreasonable things of the new alicorns.