But I've never had a problem with a screen capture of a boarding pass, and I've found the Apple wallet to work as well. The airport kiosks are good for printing out the pass when you check in, or even for just printing if you checked in on-line 24 hours prior.
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.
I prefer to use mobile boarding passes so I don't have to waste paper, and don't have to keep track of yet one more thing while traveling. (I'm already bringing my phone, so I have to keep track of that regardless.)