The Determination injections reversed the state of “fallen down” for the monsters in the Lab.
Heck even the remains of monsters can be brought back, as seen with Flowey, with Determination and a physical form to anchor to.
And I think I know why.
My body…It feels like it’s splitting apart.
Like any instant…I’ll scatter into a million pieces.
Deep, deep in my soul.
There is a burning feeling I can’t describe.
A burning feeling that WON’T let me die.
Monster SOUL’s have Determination in them.
But unlike humans who have bodies made of physical matter, and whose Determination makes their Souls persist after death…
Monsters use their Soul's Determination to maintain their bodily form.
That’s why even normal monsters like Gerson can live for a millennia without ever “falling down” and why monster Souls don’t last long after the Body is destroyed.
Age doesn’t cause “falling down” because the age pool of monsters is at such a wide range going from Snowy’s mom to Shyrin’s sister.
When monsters loose Determination it compromises their physical wellbeing.