Except that then there's zero point to their human partners and the spell books. Like maybe they can get new spells, like Zatch and Kanchome did, but I doubt they'd train to now use their spells at all, especially since Naga being drained was draining the energy directly from the Demons, so this is far more than them losing their spells
in the manga world, there would always be an explanation. might not be a good one, but explanation nonetheless. it's called "Deus ex Machina". or at least it used to be. IDK what the recent generation called it now. plot armor, maybe?
anyway, back to the topic. of the top of my head, i can imagine that for humans, it might be how to make/design a new spell, and for the demons, to actually make that spells. kinda like architect and engineer kinda thing. but if before, all spell originates from the electric dragon thingy the antagonist want to take, now they have to make it themselves.
maybe the human would have to understand what exactly is heart energy, and how to harness them. not just meditate/rest to refill it. maybe it would be how to up the quality, but less quantity ot vice versa.
and for the demon, how to harness the energy/spell better. imagine if Gash can cast Zakeruga from the back of his head to deal with sneak attack. or zakeru from the bottom of his feet to give a speed boost.
or maybe it'll be like Digimon, and the Demons and their partners will fuse.
so many potentials.