Is it time now to make it compulsory to have a ticket before travel. I know very small stations don't have ticket machines but now that nearly everyone has a smartphone it would make things simpler if you had to buy an e-ticket. Less work for guards, and less fare evasion or unintended non paying of fares.
No, it isn't time.
Just as it it isn't time (and hopefully never will be) to insist on payment in advance by smartphone for so many other day-to-day transactions.
Some people like paying that way, fine, keep it as an option, but don't force it on everyone else.
How is that chain of coffee shop (singular) which refuses to accept cash getting on? Has it achieved world domination yet, or been simply forgotten about?
If simpler is your ultimate aim, make train travel free.
If a TOC, RDG or Dft wish to provide everyone who requests one with a free smartphone including free data, in order to enable them to meet your requirement, than I still see only minimal merit with your proposal. Perhaps you could put someone on each station in order to offer use of their smartphone. You could give them a room to sit in and call it an office for tickets, or something similar.
Plus of course free technical assistance for those unfamiliar / uncomfortable with the technology. Also assistance for those unable to use a tiny viewable screen and touch screens due to infirmity / disability.
In addition guaranteed mobile signal at every station and chargers for those who have been out all day and lost all charge.