Two interesting points here.
Firstly I think there are always going to be exceptions / people who can't or won't use e-ticketing. However just as mobile banking was unheard of 20 years ago, and its now increasingly difficult to find branches to transact your business (I can't remember the last time I was sent a cheque, for example), its now the absolute norm. A smartphone has become the primary device most people have with them and use during most travel. So some method of keeping your travel pass on your smartphone seems to be extremely wise, and eTickets (I've used Avanti and Caledonian Sleeper's in recent weeks) seems pretty simple and well implemented. I suspect it will become increasingly the norm, and the amount of station concourses occupied by inefficient, clunky TVMs and ticket offices will naturally reduce over time. I am surprised given all the back end systems exist for eTicketing, that more TOCs don't incentivise its use with a nominal discount. I suspect that would rapidly drive growth and reduce costs for them.
That said, in this part of the world, it amazes me given the cost penalty for using a paper ticket on Glasgow Subway, and the widespread marketing of the free smartcards, how many people still queue in a line at a poorly staffed ticket office missing trains to collect their disposable ticket.