There is no reason why the elderly, disabled or low income people can not use a debit car*d.
*
This is woefully ageist, ableist and privileged.
-Card only services rely on use of apps/websites to monitor transactions and balances - a significant number (not all, admittedly) of elderly people either do not have a smart 'phone and/or do not have access to the internet. Yes, in another 20/30 years time that may no longer be true, but as it stands at the moment, that's the situation.
-Using a debit card is too abstract a notion for some people with learning disabilities to process. Also, even if you remove the need for an app or website to check transactions/balances, there is the need for a mental tally of purchases, so you know how much of the £20 you have left - this might be impossible for some people. It doesn't mean they shouldn't be independently accessing shops, or even if they're supported, it shouldn't mean they can't pay themselves - spending £12 and being left with £8 in your hand is much easier to process.
-People on low incomes sometimes rely of cash to know exactly how much money is left and where it is going. Transactions don't always appear instantly on debit cards, and when you're working down to your last 50p, it's easier to not risk going overdrawn by paying cash is possible.
I also believe that teaching children about money is best done with cash - physically holding 3 £1 coins at 5 years old is much easier to understand than having a card that 'represents' £3. For older children, yes, debit card education is vital, but for the beginnings of fiscal education, physical manipulative (coins) are vital.