Personally I really hate this idea, not because i'm against the idea behind it but because I think it's the wrong approach. We really need a more involved spawn conditionals system so people can setup their events in an intelligent manner.
Perhaps a system where npcs can spawn and despawn based on certain conditions set by the quest system. Ex: In this case your boss is engaged and you have it set boss_one_engaged flag for the zone and all npcs that aren't essential despawn. Or perhaps you have a linear zone and only once a boss is killed the trash and next boss will spawn. etc etc.
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