But we already know the purpose he served--to point out the weakened celestial currents. Even if a hypothetical replacement isn't as knowledgeable about celestial currents as he is, they can still point she or he in the right direction from the beginning. Of course, someone else might have figured out that the attack was coming from outside the planet, as the four working together did in Elpis. Curious that synergy didn't happen again, huh? Almost like Hermes was sabotaging from the very beginning...