Yeah, I mean, couldn't they just wish for it? For immortality or to become demons...
They will just wish for immortality from the demons ( it’s obvious their demon bf will persuade them to do so) > end the contract > demon return to demon kingdom > those 2 can redraw their lover sigils to re-summon > make new contract
All happily ever after
I'm more worried about how much damage they've done to their souls. If this followed any of the myths about summonings, they're using up a lot of soul energy.
i wish the mangaka make a sequel about gorou and hiriki so that they can be immortal... i still can't accept that they would eventually die (⊙…⊙ )