No-one objects to an airline ticket being issued as an e-ticket (which can be printed or dispayed on a phone).
I think you're confusing tickets and boarding passes. A ticket confirms you've paid for travel. A boarding pass means you've also answered the security questions, entered your passport number, have your seat confirmed, been asked if you want the meal deal etc. 15 years ago budget airlines were saying "We're a ticketless airline", what that meant is they only needed your name or booking reference to retrive your booking and print out your boarding pass. Traditional airlines had issued tickets, that were exchanged for boarding passes. Now the majority of airlines don't have tickets.
With regards to boarding passes being issued as pdfs or in the airline's app, airlines like Ryanair allow that option for no additional charge. For your boarding pass to be printed at the airport they charge you an eye watering amount. Even if you don't have an printer or smart phone, it would make sense to visit your local library and pay whatever they charge to print out the pdf boarding pass, than to go to the airport without one.
Airlines don't allow you to check in online if your departure airport doesn't accept pdf/mobile boarding passes and you can then check in for free at the airport.
There's no financial benefit to rail passengers in being able to print your own pdf ticket at home. You're saying the train operator money but they aren't passing any of that saving on to you. If you got a 50p discount over the ticket office rate for using an e-ticket then it might be a different matter. That might not go down well with TSSA though!
its even less difficult if you have a charging cable. I've charged it on trains, but never looked to do so at a station.
With Northern you can't be sure you will get a train with USB charging points and even if you get a class 150 with them it might not be that easy to charge if you're not in a window seat or if you're standing.
I once made a Megabus journey of a few hours and despite the coach advertising USB charging and WiFi, neither feature was operational. I would guess that could happen on a train too.