The snappy retort to that is, for the same reason we don't charge customers in wheelchairs extra for taking up more space, and for the cost of the portable ramps at stations.
The more polite way of putting it is that my late grandmother had to move from her own flat to a care home partly because some of the services she needed were becoming less and less accessible as they went "digital". Age isn't a disability, but it is a protected characteristic under the Equality act - yes I know the legal protection that provides is quite limited, but from a moral perspective, discriminating against people who didn't have smartphones when they were young and would find it a challenge to learn now seems wrong. It was not a question of whether she was happy to go digital, she wasn't able to.
Another example why I personally am not going to use m-tickets or anything phone related for tickets any time soon: did you know that if your employer uses Microsoft Exchange as their e-mail service, and you install their app to read your work e-mail, then you're giving them the ability to lock and wipe your phone remotely? Meant for if you report it lost or stolen, of course. That's just one of dozens of things that could go wrong, but would ultimately leave me liable if I'm not able to present a valid ticket to an authorised official on demand.
Over here in Bristol, First bus introduced m-tickets supposedly to make boarding faster compared to fiddling with cash. You can still buy a paper ticket with cash, but it costs 50p more for a single or £1 more for a day ticket. They didn't make tickets cheaper when they introduced this, even though it makes boarding more efficient and saves on paper - the m-ticket is the same price as before, paper is now 50p extra (even though it's till roll with a QR code, not magstripe). Presumably the m-ticket also gets them lots of resellable customer data. The thing is, you can also buy with your contactless card, which solves the cash problem - but that still gets you the higher tariff, you have to use the stupid m-ticket system or a smartcard that can only be loaded at the central bus station if you don't want to pay extra. So I don't trust the industry that a move to e-tickets won't be used to increase prices and gather all kinds of extra data without most people noticing.