Why should we pay for printer cartridges
You don't have to. In due course I suspect tickets purchased from ticket offices or dispensed from TVMs will be e-tickets.
Do e-tickets allow for the purchase of multiples of tickets for the same journey to be stored on one smartphone?
Yes.
It does differ from an airline boarding pass in that in the event of some failure (etc) you can't get on the aircraft and thereby incur any "non-payment" punitive action; you may even be able to get the boarding pass reprinted at check-in. And once onboard, no further ticket examinations take place. In other words, there is no risk associated with use of electronics. Whereas you can board a train and unwittingly be able to show your ticket, and that may incur punitive action. That is the issue I have.
That can be a problem but so can misplacing a paper ticket once you've boarded a train (something I have managed to do in the past!). E-tickets are effectively a barcode and how you present that barcode for inspection doesn't matter.