I prefer a paper ticket because when I bought something described by TransPennine Express as an e-ticket, I wasn't permitted to print it out or show it on any device other than the phone associated with that account. The ticket in question was a sale ticket, so had to be bought on their website.
On another occasion, someone I was travelling with had an Off Peak Day Return which they wished to excess to use at a time of day that would otherwise be restricted, but the ticket office refused to issue any excesses for e-tickets, saying they'd need to buy a new ticket. In the end, the person I was travelling with was given permission to use their invalid ticket without paying extra.
A further problem has arisen for me personally in that e-tickets are frequently not recognised at gatelines at intermediate stations, this would make 2D code only tickets quite unsuitable as season tickets.
Finally, some sites still try to sell m-tickets, which are very poorly implemented and theoretically take away some of the traveller's rights, while other sites still sell old self print tickets, which I find really inconvenient because of the need to access a printer.
Finally, if one buys an Advance ticket online for ToD and then wishes to amend it at a station, say, 10 minutes before the deadline to do so (when the train departs), that can be done. However, a station will not be able to help with an e-ticket. So how can you convert your Advance ticket to an Off Peak Single (say) on payment of the £10 fee if you only have 10 minutes to go?
A final reason is that I have seen many situations where a member of staff creates a confrontation with a customer about the validity of the ticket because the staff do not have a scanner. This has ranged from the claim being made that the ticket isn't a real ticket, to the ticket isn't valid on that company, to the ticket is specific to the organisation from which it was bought.
To put it another way, the range of products becomes too confusing.
While confusion remains among staff and passengers about consumer rights under these circumstances, I am unlikely to adopt the new technology fulsomely.