Proof

bitcoin reserves are auditted as held with full confidence any instant. Gold reserves depend on confidence of last auditor last time it was done, and it would be national security to lie that the reserves are still there.

energy-rich actor could shoot for a 51% attack without sending a single troop across a border.

a 51% attack has a small chance to undo a spend that was done just in last hour

It doesn’t matter where something physically is, but rather how do the parties to the transactions understand the exchange.

IOUs are not as good as money, IOUs based on a "trust me bro its still there asset" is not as good as IOU based on bitcoin, and an IOU based on fractional reserve inflatable currency is far worse than asset based IOU.

So with your passphrase in your head (and no new holes) you leave. Where do you go? Somewhere where there are other nerds who will take your distributed ledger units in exchange for the necessities of life

Trying to sell gold coins to pawn shop or gold dealer gets a bad price relative to IOU trading. Global bitcoin exchanges often have the best prices, but nations you'd want to escape will tend to have problems that put a premium on bitcoin because of its escape advantages over gold. The argument that you need to convince bakers in your new country to accept bitcoin is not real. But its easier than gold, tulips, or other currency, or land deed in civil war country. Replace baker with someone on craigslist, and its still easier.

deflationary, which makes it a lousy medium of exchange. You’re basically saying that the supply of gold (or other material mediums of exchange) can increase, as if it’s a bad thing.

You're arguing here that paying with Bitcoin is bad for the payer because value is certain to go up. That means it's good for the buyer/receiver, which makes it easier if you need something physical/consumable from your "wealth".

This isn’t a common problem in stable nation-states

Bank panics/failures are frequent enough that the system relies on bailouts for them. Deposit insurance is something customers pay as a hidden fee, but often taxpayer bailouts supplement the failures. Settlement "failure" happens often too. "Trust me I'm solvent bro" is a bad system that has and will regularly fail.

If the USA goes Mad Max even put aside the computers and network access needed to maintain a blockchain (other countries exist, etc), what is the impact of the evaporation of all the USD which were use to acquire BTC on the exchange rate for said BTC in the still-existing nations?

USD has no relevance to value of bitcoin. Mad max with internet access can trade for go juice or guns and bullets. Gold would tend to have very low value because Fort Knox is indefensible. Preparing for global mad max is different than preparing for local chaos. Communal leadership skills you can use to convince building fortifications and somehow secure food access is needed for communal survival. Wealth onto you would come from an ability to impose hierarchical tributary power system.