The CMC would've seen them at Granny Smith's age with no cutie marks, Applejack would see the entire orchard in ruins, Rainbow Dash would either have the Wonderbolts rejecting her or having her wings clipped, Pinkie Pie would either be her losing the right to inherit the bakery or eternal misery, Fluttershy would see a slaughter of cute animals, Big Mac would see himself eternally crippled, Rarity would see herself fired as Ponyville's master designer.
Unless there are fans that hate Spike with a passion, and I'm sure there are, I think if Twilight ever did do this, she would become that most hated character in the show. Which would be a really bad mix since she's the main focus.
So by that logic, if she fears failing a test from Celestia more than losing a baby dragon who's practically her adopted son, does that mean in the end she's a selfish nag who values her own success more than caring her friends and Spike?
That's just cold, and anti-thematic to the FIM setting. So logically failing Celestia's test couldn't be her greatest fear, maybe before Season 1 when she had yet learned the magic of friendship, but definitely not her greatest fear by Season 3. If anything, the "Return of Harmony" episodes shows just how much her friends meant to her - whereas she panicked over being tardy to Celestia an episode later, losing them to Discord's brainwashing in "Return of Harmony" psychologically BROKE her.