I’m sure someone has said this before, but it wasn’t Celestia that banished Nightmare Moon- it was Daybreaker. Banishing her sister is a pretty extreme measure that I just don’t think Celestia would ever take, even if it was unavoidable, but it’s something Daybreaker would do in a heartbeat. It’s why Celestia is so torn up over Luna’s banishment- she made a stupid mistake and let Daybreaker take over, and DB’s arrogance is what pushed Luna over the edge into becoming Nightmare Moon. DB won that fight, but Celestia was horrified when she figured out what happened, so she swore to never use the Elements of Harmony again, hence the need for Twilight and friends to save Nightmare Moon.