So it didn't take me long after watching the season 2 finale to realize that Belos' plan to use the draining spell to kill everyone on the Boiling Isles and "protect" humanity was utterly a waste.
Now we all know the plan. But here's the thing: The draining spell only affects those with sigils. We know for a fact that the students at Hexside for the most part didn't have one with a few exceptions. We also don't know how many other schools or places on the isles outright resisted the efforts for the Day of Unity. There were likely thousands or more that didn't have a sigil by that point which means a ton of citizens would have survived. It would have been a long and disorganized process but the isles would have rebounded. And afterwards, heaven forbid someone figure out how to make another portal and want revenge.
And that doesn't even bring into consideration the many other civilizations like the Titan Trappers who did not have sigils or know anything about that. There are likely many other kingdoms, countries or groups just living life that would be unaffected by this plan. Belos seemed like he was just giving a final middle finger to the Isles, causing heavy, but not unrecoverable damage before he left.