I usually print a boarding pass at the airport kiosk — I always have luggage to check anyway, and there's then no concern if my battery dies etc.

But at the automatic gates to security, and at the boarding gate, I think the majority of people having trouble scanning their boarding pass are using a phone. For those elderly people who can't scan a paper boarding pass, the staff can immediately help — but they're much less able to help scan a barcode on a phone, which might be set too dim, be turning to landscape mode, etc.