Being temporary is alright.
And I don't think your rationale really holds up. Just because light is "stronger" than darkness doesn't necessarily means angels are more powerful than demons in every aspect, especially if you consider demons to be fallen angels. Presumably, fallen angels aren't any less powerful than regular angels, they just use their powers for evil.
If you are using the rationale that angels are so much more powerful than demons, then there are 2 problems:
1. They are basically overpowered and unstoppable, since the only thing that could stop them would be a more powerful angel.
2. There is no reason they shouldn't have been able to manifest in the world directly and kill all the demon and make everything right in the first place.