I used the vague "nation state attacker" term on purpose. Once the old system breaks we will enter a chaotic period where anything could happen. Everything will change rapidly. It is just not possible to make high confidence, evidence based predictions about what will happen during and after this chaotic phase. Right now the US and China are the only credible "nation state attackers" but that could change rapidly. A lot of people are scared of a malevolent AI killing all humans. I don't put much credence in these doomer theories but I think "nation state or equivalent" covers this scenario too.

It's important to distinguish between an intentional hard fork like bcash and an unintentional one like a network split would cause. With bcash, people were able to pick which side of the fork they supported or do nothing and see where the chips fall. They knew not to broadcast certain transactions that could get replayed on the other fork, etc. An unintentional hard fork will not be orderly. People may not find out about it until their side of the fork experiences a blockchain reorg and their recent transactions get reversed. This scenario is very bad for bitcoin as a whole. Confidence and price will plummet immediately, bitcoin businesses will lose money and many will fail. It will set back the entire bitcoin movement many years.