That little bit about the Demon being "Mana's Personality" rings really true for me
If you remember back to when Laios first became lord of the dungeon and wished "For a world without demons" it responded with "So you want to use Magic to bring back your sister but make everyone else live in a world without it?" Even at the time I thought that was a weird sentence, but chalked it up to the Demon's honeyed words. It also more concretely links the Winged Lion knowing Mithrun. The analogy was that every demon is like a hair all coming from the same creature, but I didn't quite expect that "creature" to be magic itself.
Even though we know that
1. Dungeons Tapped into another world of infinite Mana
and
2. The Demon apparently came from that world
There wasn't a concrete reason to think ALL magic came from that world, but it seems like that probably is the case.
Mana came from that world, spread, integrated into life and biomes, devoloped a consciousness as the demon, and would have been gone forever if the demon was truly killed.