> it's not a reasonable negotiation tactic as following through on the threat would accomplish nothing - and both sides know that.
It's effective, because the other side is forced to bet their own lives on you eventually accepting defeat and agreeing to die, so that they may live. It's not an easy bet to make and stick to.
All cooperative moves are indeed better than suicide, but when we're at the point everyone wants to get to NZ, we're way past the time of cooperative solutions.
There is no indication that it will be even an effective tactic in the situation described. Even in simple cases, the other side could assume it might intercept some of nukes, some might not work, it might get some with conventional weapons on the ground - it will never be so clearcut (even ignoring any preemptive moves).
Also, assuming that there are only zero or negative sum moves left is a really big assumption.
It's effective, because the other side is forced to bet their own lives on you eventually accepting defeat and agreeing to die, so that they may live. It's not an easy bet to make and stick to.
All cooperative moves are indeed better than suicide, but when we're at the point everyone wants to get to NZ, we're way past the time of cooperative solutions.