That's not exactly it.
Source in itself is a form of energy. Therefore it follows and applies laws of physics.
Energy cannot die, it simply changes forms and dissipates but never dies.
Source, when consumed from normal sourcerers turns them silent, as the lore explains, removing Source from a living being is worse than death, that's because there is no safe way to remove it, the trauma shock is so great that people become thralls, mute and obedient, but they need a direct order to do anything. This is confirmed early on on 2 occasions:
-Silent Monks don't necessarily attack you when you kill the butcher in the fort dungeons, they never get an order to attack you unless during combat he runs off close enough to one of them to order them to attack you.
-Silent Monks in the fort harbor never get an order to attack you so they don't.
In addition, Dallis can turn some sourcerers into Shriekers, a form of wraiths inside living shells.
It's not like people from whom you remove the source and turn them into a silent monk are completely dead.
Theoretically they should be able to return to normal if some sort of therapy was applied, or they could create new personalities over time.
Also purging source from rivelon is not killing all the people, keep in mind not everyone has source.
Problem is, we can't be sure that the void will not find another way in, and the only way to effectively fight with the void is with source.
Also keep in mind that the other games in the Divinity franchise are placed much later in time and there is VOID and SOURCE in it, which means that even if you seal the void gate, it will find another way, and if you purge source nature will find a way.