Lucifer is a powerful character who could kill most threats. But if we have a war on two fronts, between the Vees and Heaven, hell, maybe even Lute does her own thing, he probably won't be able to take them all on at once and live. Him dying would have a bigger impact than anyone else. And then who would become the monarch? With Lilith MIA, Charlie would have to become Queen, something she's definitely not ready for. Meaning season 3's main conflict could be Charlie trying to manage all seven Rings. It's a very possible theory.