An end to return fares might lead to great fare increases as on longer routes the difference in fare between a single and a return isn't that great - before taking into account potential issues such as railcards, break of journey, and passing multiple barriers on one journey - which could trigger 'new journey'. E.g. if I travel from Woolwich Arsenal to Kings Cross on contactless and go via the underground, I get charged two journeys, while if I go thameslink, it counts as one journey
That shouldn't be happening, where are you changing?
It represents a potential revenue loss for the operator and perpetuates exactly the sort of complexity most 'normal' people want removed. Contactless is simple - point to point charged at exactly the amount the operator wants to set as the fare for that journey.
Do you feel that the choice between paper ticket and e-ticket for the same fare on most TOCs at present perpetuates that complexity? Really?
The complexity people don't like is that the fares themselves are highly complicated - singles, day returns, period returns, TOC specific tickets with no significant discount just for an ORCATS raid etc. If you switch to having only Anytime Day Singles, Off Peak Day Singles and (optionally) Super Off Peak Day Singles, with clearly publicised time bands for each, and you being able to use paper*, e-ticket or contactless for each, what's complex about that?
Break of journey and precise routeing are matters that only really concern people on here. Most people want to take a direct train from their intended origin to London, and so they are the main people you are aiming at. The niche situations where double backs aren't allowed but sometimes make sense (e.g. Bletchley-Euston) can be dealt with by tweaking the fares, e.g. by introducing a Milton Keynes Stations group and setting the fare the same for all of them, or just ignore it because basically nobody cares. For example people already travel Euston-Bletchley via MKC occasionally, and as long as they don't try to use Avanti and don't try to pass the MKC gateline, nobody cares. And even if they do try to pass the gateline they just get told to go back, I've seen it.
* I do think paper tickets should be e-tickets, i.e. if you buy a ticket from a TVM or window it should be barcoded and verified by scanning. That however is a different debate.