Wow, this really is a marmite subject which we regularly seem to debate and go round the same circles.
To summarise:
Paper tickets - need to be printed at a station or a TVM. No power or technology required but lose it and it can’t be replaced.
E-ticket- a barcode that can be shown on any device, a smartphone generally being the device of choice. It can also be shown on paper if the holder chooses to print it out themselves. Risk that if the smartphone loses power that the ticket cannot be shown.
Both systems have their advantages and disadvantages. I suppose the belt and braces approach is an e-ticket displayed on a smartphone with a paper copy in you back pocket just in case