The problem with this is you can still lose to the other players' AIs and the other players can still rejoin and come back, so that wouldn't really make sense as a feature.
Ah, I see
I would suggest a change to this then: Allow victory to be declared if the human player controls at least 40% of the territories, because players can and do lose to enemy bots.
A player should be able to surrender. But the winner should not declare victory.