Comments of chapter undefined of I Farm Demons : Unholy Necromancer In the Apocalypse

I found a plot hole. If soul keepers die after 15 years and the last soul keeper reincarnated Xylon and Xylon is 18 years old, does that mean there was no soul keeper for 3 years. Also isn’t Xylon and Eva the only sinclairs. Wth happens when the sinclairs die out. IF Xylon died in 15 years who would even replace him?