The priests wouldn't let her kill Mammon though.
Since he is naturally immortal, it makes sense that he wouldn't try to keep young because he wouldn't die of old age. Meanwhile, the rest have some sort of method to perpetuate immortality, Mottom has the fruit, Gog-Agog is apparently some sort of hivemind of worms (makes me think Sid Meiers Alpha Centauri kind of thing), Jagganoth is immortal due to the iron angel feathers stuck in him, Jadis is in some state of suspended animation. According to the wiki (as said in one of the tumblr pages), Solomon David is apparently immortal due to martial arts practices, and it's not clear what makes Incubus immortal. Although that blood pool at one point could be a hint as to what makes him immortal.
One thing that I'm wondering is if Mottom actually
needs any help to be immortal. Having the secret names of YISUN isn't literally the same thing as being all powerful, but it sure seems like the next best thing. Mottom has a seventh of that power at her beck and call; and the particular names she has give her power over living things (particularly plants). She turned a man into a tree and another into flower petals without breaking a sweat. Seems to me altering her own form at will or creating another should be little more difficult. Or duplicating the fruit of youth simply by her own will.
We know that the keys' power is best wielded by those who just
do things without permission or second thought. Mottom never broke free of fear or of her husband's influence. She practically said as much that she hates herself and her position. Living or dying, or finding a suitable replacement and fucking off to live somewhere else, all should have been within her power if she would just pick one with confidence and stick with it.
I think Mammon too is not adept at wielding power, but in a different sense. The bit that sticks with me from his intro is that he cares about NOTHING but the count. No friends, no other goals, just the dragon and his horde. He may have the confidence to wield power, but he lacks purpose. Its like he had a magic sword of legend and hung it over his fireplace. Doesn't matter how sharp you are, can't cut anything if you don't take a swing.