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I understand you here. Certainly, when the internet is gone (Iran, Venezuela after earthquake) and you want to "buy with bitcoin" right then, it's not useful. Doing an satellite tx or "on paper", though technically possible, isn't really practical in an I-need-it-now situation.
To be fair, the same is true for most of the way I purchase things now: credit card or debit. When things go stone age, we go back to sticks, rocks, and chickens.
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.
Bitcoin is useless if the internet is shut down. I'd love to hear what bitcoiners in Iran were doing when they shut the internet down for months.
There are of course other ways to broadcast a TX like with Block streams satellite. People are also working on offline payments using ecash and I saw another on SN recently called Kagikai but it hasn't got much attention yet.