1000 years is a long long time to make any predictions. Predicting 10 years into the future is hard enough.
That said, a thousand years is a long time to research a Death Star I suppose.
For example, what would be the reason to nuke the Amazon, the Arctic, or the Himalayas? How much of Africa would be targeted? What percentage of the US Midwest or Siberia do you suppose would actually be irradiated? What about islands in the Pacific? Is Easter Island going to dosed in lethal radiation from fallout tens of thousands of miles away?
https://www.quora.com/How-destructive-is-the-worlds-entire-n...
for MAD to work all you need to assure is that you can damage the opponent enough that their power structure will not survive, not to destroy all of their people/land/capital.
vacuum decay
a gamma ray burst probably won't happen even once in our entire galaxy
We are due for a gamma ray burst. https://youtu.be/8HvwNOSnveE