The grid doesn't have to go down globally for your crypto to be useless, it just has to be down where you are. This is already becoming a real problem around the world as even fiat moves digital. When power goes out, stores become cash only, or they close. You can't go to an ATM, you can't withdraw from your bank, all you can do is argue with the store clerk, and ask someone to loan you money.
That's why crypto can't replace fiat. That's why credit can't even replace fiat, though it's been trying to for the last 40 years or so.
We do, technically, have the ability to get around this now, by distributing the internet across battery powered devices, but we aren't implementing it yet, and probably won't for quite some time, because it will cost the people with the server farms hundreds of billions of dollars a year in lost revenue.