Metaphor: Refantazio pre-launch theory
The Archetypes and humans are connected, they even might be two sides of the same coin.
Their looks are the complete opposite: Mecha-like, artificial form of archetype, representing the heroes of old, and the fleshy, Boschesque form of Humans.
But when the protagonist discovers the power of archetype, during his face of against human it turns out, they both have share similar skills: wind magic and healing.
Stated in interviews, one of the themes of the game is anxiety, and how it affects you.
The awakening to archetype power happens in dire circumstances, during great emotional turmoil, the user has to rip their heart out and restate their ideals and steel their resolve, even against impossible odds.
Maybe the people (or only elda?) can turn into human when they let the desperation and fear overcome them, and make them give in to their primal instincts, and lose their humanity along with their ideals.