He is skeptical that there will ever be enough mortals to fill out those ranks, and skeptical of this purported better service they'll give Him. Nethys purportedly cannot see decades or millennia into the Future, with His earlier-founded selves not much further ahead in time.
How, exactly, is this trade-good meant to be assured, to Asmodeus? For merely probable trade-goods come with a probabilistic discount, and the probability of this one has been supported hardly at all. Here is Him legibly exhibiting that His skepticism is real and not simply a negotiating tactic -