Surprised mobile boarding passes haven't changed to ticketmaster-style passes that leverage the phone's NFC capabilities. Seems like that would be faster to scan at the gate and prevent duplication.
But it also places undue financial burden on passengers. Not everyone has an NFC-capable phone, and the workaround (paper tickets) is what we already do.
As an aside, Ticketmaster is miserable to deal with.
Not every phone has NFC capability. Every smartphone has a screen though, making scanning a barcode more accessible, and able to fallback to printing if the phone runs out of battery.
The last time I attempted to use a mobile boarding pass, the app failed to load the pass¹. I later figured out why: the airport's WiFi was MitM'ing the connection, and instead of letting an AJAX request for the boarding pass through, it wanted to serve ads. A paper boarding pass won't try to transmute itself into an ad at the moment of "Papers please."
Nobody wants to be doing that in TSA, of all places.
And if you're checking baggage at all, then you more or less get the paper pass for free.
¹…you can screenshot the pass to work around this. But now we're getting a bit closer to what the guy in the OP did!
As an aside, Ticketmaster is miserable to deal with.