Sargeras was speechless because Roy's words hit the nail on the head.
With Sargeras's strength, even if an Old God Devoured a planet and grew into a complete body, he still had the ability to eliminate it. Regarding things that he could deal with within his abilities, even if he were vigilant, he wouldn't feel fear. Only after he saw with his own eyes that an Old God had corroded and corrupted a titan world-soul could he find the feeling of substitution.
It wasn't until this moment that Sargeras truly realized that what Roy had said before was true. A powerful force was corroding world-soul titans through these Old Gods!
Uneasiness brought fear, fear brought terror, and terror brought panic. Sargeras clearly realized that if these Old Gods were allowed to continue spreading, all the unborn world-souls in the universe would become targets, and the titans would no longer obtain new people. The corrupted world-souls would even turn around to deal with the titans.
