Nobody who cheers "mobile tickets" has ever watched a guard or ticket inspector do their job. In the UK, a guard with good eyesight and a sprightly, cheerful manner can almost run through the carriages shouting "tickets please!" and see, with a single glance, all s/he needs. Then she gets to the muppets with phones. They faff, fumble and fuss. They scowl at being interrupted watching videos, browsing social media and…
My local station doesn't sell tickets. It has no office. I can buy an advance then drive miles to a totally different station to print them out or just buy it on my phone even on the way to the station and be done with it. I guarantee you buying from the ticket inspector is slower than showing them my phone.