Risk assessment covers both the probability of something happening, and the consequences when it does.
Yes, my phone dies rarely, but it does happen. And it happens even when I'm not to blame by scrolling through Facebook all day
But take that small risk, and multiply it by the revenue protection consequences to me if my phone dies. That's a combination that I think is too strongly weighted against me, in favour of the railway, so I'm not prepared to accept it. The railway could fix this by giving people the benefit of the doubt, or allowing alternative methods of validating my ticket (quoting a booking code, giving a name and address, etc.) in the event that my phone does die. If, as is being suggested here, I'm just unlucky, and the population-level risk of a phone dying is so small, and it happens so infrequently, then allowing this shouldn't be a big problem to the railway, should it? For as long as I can be threatened with penalty fares and prosecution if my phone dies, I will always be nervous about relying on my phone for railway ticketing.
For a one-off occasional long-distance journey, especially one where I might want to spend a while planning it and looking at different time versus price trade-offs (major pain to do that at a ticket office!), I absolutely don't mind buying online, using an e-ticket, and printing out a backup in order to mitigate the phone-dying-risk. But I would prefer that the railway took a more reasonable approach to how it treated me if my phone died so I didn't have to make the physical backup.
For a daily commute (OK, maybe weekly since Covid), where I don't plan the journey in advance, printing out the backup every time is a big hassle. So the old-fashioned methods win, and the ticket office gets my custom. And after doing that a few times, you start to remember that a ticket office is really quite efficient for basic everyday purchases.
I feel that the risk of that happening is much lower than the risk of my phone dying. The consequences to me in both situations (from a revenue protection perspective) are identical. Therefore risk*consequence means that I'm happier using paper tickets than I am a phone

Of course, the railway could change that balance by adjusting the consequences.