The obvious solution surely is to amend the TfL gates so that they accept e ticket barcodes. This would require funding from DfT, as TfL don't have the funds to do it. If DfT really want to abolish card tickets, then they need to find the cash to do it.
An alternative might be to have machines that would read the e ticket and issue you with a card ticket purely to get you through the tube. But again, someone would need to pay for them, and better to pay for a proper solution.
This is obviously
not the solution, because barcode tickets take around twice the time to pass through gates as contactless do (rates of around 15 per minute and 33 per minute from studies that have been done, from memory). The reduced flowthrough means that TfL can't meet their safety cases. It
might be the solution for a select number of aisles at only LU stations that are adjacent to London Terminals, if space permits, there is adequate signage, and customers are communicated with so that they know what to do. It is very expensive to add barcode readers to gates by the way and, given the quantity on LU, the case would have to be very good.
In both cases it would enable the retirement of CCST as a fulfilment type, and the consequent removal of magnetic readers from LU gatelines though. So a significant saving there over the long term.
The best solution, from both a customer and a cost perspective, is to not enable any LU gates with barcode readers, but instead to remove the "LU transfer" validity of tickets that pass through London, reducing the price of those tickets by £3.00 / £2.00 / £1.50 (adult / railcard / child; double these for return tickets), and changing the route code to "Not valid on London Underground or DLR" (or similar). They would still be valid on Thameslink, where appropriate to the route.
In this scenario, no through tickets would be lost; the message to customers would be very clear and simple; TfL would not have to spend millions on barcode readers; magnetic stripe readers could be removed; and most people would actually be better off (an adult would typically see their fare reduce by 60p). I realise that not everyone would be better off - railcard holders, especially in groups such as with a Family & Friends railcard, spring to mind. But it's impossible to change how things are done and make everyone better off, unless you increase subsidies massive (unlikely at the moment!).
Travelcards would be smartcard-only products (anyone without one will need to get one, which I do realise is not going to help their sales), and fares to London Zonal destinations would either also be smartcard-only products or, perhaps, cease to exist.