I do think a lot of people are seeing online-purchased eTicket vs station-purchased physical ticket as a binary thing. In reality it won't be; you will be able to purchase most tickets from whichever of these you prefer.
As now, most stations will have TVMs; and some (though probably fewer than today) will have booking offices. Eventually they will all issue barcode tickets, rather than CCST (orange magstripe tickets) - but they will still be physical tickets. Techincally they will be Paper Roll Tickets (PRT). I appreciate that some forum members don't like these, but they will help to reduce costs in several ways, as well as being recyclable. Where (as now) stations don't have either a TVM or a booking office, then you will be able to buy a physical barcode ticket on the train. No-one is going to be forced to have a phone or internet access to buy a ticket!
For those people who are comfortable buying online, and using the barcode ticket on their phone (or a print out of the barcode ticket) to travel with, then this will (of course) continue to be an option. All products (ticket types) and all routes will soon be available in this way. Technically these will be either eTickets or (mostly for seasons) sTickets - but in both cases they will be barcode tickets. You don't have to do this though - you remain free to go to a station and buy your tickets there. As some members have observed, buying online and using a phone is now by far the most popular method of ticket fulfilment for customers.
People will also be able to use contactless pay as you go on a wider part of the network; and ITSO smartcards for almost any journey / ticket type.
I expect that ticket on departure (ToD) will be dead within a few years. The old self-print tickets format is being discontinued (in a couple of months). And hopefully m-tickets will be discontinued as a format in a couple of years.
Indeed, and given that rail travel (even allowing for the recent drop) is now about as busy as it's been at any time before the last decade.
As such, without the increase of ticket sales options this wouldn't be possible.
Now some would like this to be only via ticket offices, however given the way the ready of the world has been going with online sales and mobile technology then it's no surprise that the railways have followed suit.
A really handy thing, if we are going to have a single ticketing site and account, would be to be able to log into your account at a TVM and print. You'd then be able to get a copy of your ticket even if you'd had everything stolen.
I've previously suggested that you should be able to present your back card at a TVM and be able to access most common journeys, print pre booked tickets, etc. Being able to print e-tickets could be a useful feature to add to that list.
That could allow you to book a ticket which you can both store on your phone but also have a physical copy which you collect from the station if you so wish (which could even be a station not at the start of your trip, although you'd only be using the e-ticket until you could collect it).