If the internet went down, it would come back at some point. Satellites etc. for more coverage. Broadcast the transaction to the ledger then.

Meanwhile, use temporary offline solutions like ecash.

Or in bartering situations when lack of fungibility becomes an issue, people would revert to trust or paper contracts.

Bitcoin is money for the internet. Internet blackouts might still be a thing so gold, silver, copper, lead, food items will still have a role in commerce.