You could think of some in-universe arguments. Along the same lines as Dumbledore's talk to Harry: what if souls really do exist? In the MLP world, what if alicorns have some downside that is not yet known to Twilight? Maybe they are in constant agony after two years. Maybe turning someone into an alicorn requires a hidden sacrifice of the soul of your friends? Maybe there is a proven tendency for new (attempted) alicorns to go insane if they are not sufficiently ready for it, and for them to turn into monsters. Chrysalis, Sombra, Luna and even Discord could be examples.