It doesn't work like this.

Everyone thinks about this Mad Max, return to the stone age total collapse, but in all of the wars that happened around the world, this scenario has never happened.

What actually happens is things become the same, but shittier.

Things like fuel, electricity and internet don't become unavailable, they become expensive and limited to certain places and times. But when and where there is electricity, everyone goes to charge their phones, and when and where there is cell service, everyone goes to send messages to their family and get news updates.

People never rush adopt precious metals as a medium of exchange (they also don't rush to adopt crypto). They use local paper currency or if the local currency is completely debased, they may use dollars or euros. People stick with whatever has the lowest friction of adoption, when everything is upside-down is NOT a time that anyone is thinking about new ideas for money.

For large purchases like a car, or (say) paying to be smuggled out of the country, sellers tend to accept bitcoin because they know nobody has that much cash on hand. But it's already like that: If you're buying a yacht or a supercar, dealers typically accept bitcoin because you're not the first person who asked to pay in it.