This was the only shocking time Celestia failed. After the season 2 finale she turned into a joke. But leading up this this point she was only ever shown as god-like powerful and actually useful. She is established to be the one who raises the entire sun and the moon and she seems to just live forever, she was the one who sent Twilight at the right time to defeat Nightmare Moon, Twilight wouldn't have done so unless she was sent to Ponyville to make friends.
Twilight says not to worry about the parasprites in Fillydelphia because Celestia would solve it herself. Celestia is the one who defeats Discord, Twilight and her friends failed until Celestia intervened by sending those letters. And when Twilight was destroying Ponyville with the Want It Need It Spell Celestia shows up and instantly solves the problem.
When Fluttershy kidnaps Celestia's pet Twilight who knows her best is the one terrified that Fluttershy will be put in a dungeon. Celestia was always being bowed down to and shown to be a real proper character.
And even Queen Chyrsalis I feel like shouldn't have been able to defeat her just because apparently Shining Armour loves Cadence, Shining Armour and Cadence being the forerunners to the new direction of the show, them two being the pony version of barbie and Ken straight from the mind of the Hasbro toy department. But it all lead up to an impactful genuinely surprising "epic" at the time event.
Rest in peace, real Princess Celestia.
https://www.youtube.com/watch?v=KJI1GUcqpRc
