Would they be able to use them against us? In their current state? In the state they'd be in after we make the exodus? Think of all the things that could prevent them from using the nuclear option on us, think about them planning on a nuclear assault on us, and the difficulties that would arise from that decision, including those both before and after they had been carried out, also imagine we know they might turn to that as an option against us, and we prepare a defense against it, just in case. Even if all those issues weren't present and they dropped the bombs, think of all the ways we could render even this extreme response fruitless as a means to wipe our race out. For example, what if we disperse a lot of our people across the fuckhuge African continent? Ready to rebuild after the attacks, and even taking advantage of the opportunities left to them after the bombs had dropped? Your issues aren't insurmountable, we could easily find ways to survive it.