In the hypothetical case I'm considering the passenger obviously was able to and had previously bought e-tickets so it can be argued that they did have an opportunity to purchase but didn't take it. In my book that's evidence of travelling with intent to avoid paying the rate and the rules need to be updated. The same goes for a passenger carrying both cards and cash and not trying to use the other if one of them fails.
What if their phone is dead, or they're on low battery and aren't sure their phone will last the duration of the journey?
Every ticket site or app requires an internet connection to use. The vast majority of UK stations do not have station wifi, and mobile data is not guaranteed to always run perfectly. So you would then get cases of people asking to buy a ticket onboard because their phone wasn't loading the webpage due to a poor signal, which is more likely to happen than a broken ticket machine. This would then make it even more difficult to separate the deliberate fare evaders from those who didn't have the opportunity to buy a ticket, and bad phone signal at a specific time of day in a specific place is nowhere near as easy to prove as a broken ticket machine.