I think it is still fun. Just a different kind of fun. It shifts to a taktical game with lots of interesting strategic discussions instead of an exploration game.
Well, of course it depends on the type of game and context, it just sounds a bit like an uninteresting tool, to just have a just invincible NPCs. I think special weaknesses make every NPC more interesting. Give the NPC a weakness (eg. Superman hs cryptonite or Lord Voldemord has Horcruxes).
After humanity will die because of catastrophes due to climat change, it will not matter, who is right... only who is left.