I don't see a major problem unless there's a prolonged outage nationwide. All the critical systems have power backups. So while the ATM or point of sale terminal might be down for lack of power - and in some case the stores have backup power- there won't be much disruption. If it's 100% of the country and lasts a week or more, maybe? But the account info is probably also on servers that aren't US based.
Consider how long parts of Texas were without power, and unless you dealt directly in those areas or lived there, there wasn't much impact at all.
I do see some things shifting gradually to cash.
After years of me resisting using my debit card for anything, (very early on there were fees to use it.) I finally stopped insisting on getting cash. Like going to the bank and writing out a check to cash and cashing it like the olden times
....
As soon as I do though one of my favorite sub shops went back to cash only with a fee charging ATM inside the place.
And during the pandemic, another favorite restaurant went cash only for eat in but still takes cards for pickup. Both said it was because of the processing fees.