I love this as an option, but the AI sovs should get some kind of buff - I can see them accidentally sending their sovs into a bad situation. The Ai may not be ready for this win condition, it is used to its sovs being invincible.
On Expert and higher, the AI could just hide their sov away in the toughest fortress with a full army - that'd prevent killing them too easily. On lower difficulties, though, if it turtles its sov it won't expand quickly enough, and could paint itself into a corner.
So the AI really is the only issue: can the current AI handle the risk/reward dynamic?