There could be cases when an older-generation technology can be used to assure destruction. Say, if the new tech doesn't prevent ICBMs and nuclear explosions, both sides will still be bound by MAD.

And notice that it didn't provoke a nuclear war, and the human race still exists. Nuclear weapons weren't an existential threat until multiple parties obtained them. If MAD isn't a concern in using a given weapon, it doesn't sound like much of an existential threat.

This is a problem, but not necessarily an existential risk, which is the topic under discussion. Existential risk has a particular meaning: it must be global, whereas the US bombing Japan was local.