Is that really the case though? Oyster is indeed restricted to a much more limited range of fares, but that's surely because it has to cope with Pay As You Go and calculating fares and balances on the fly. With NR railcards, AFAIK there's no intention to use PAYG (I assume that apart from other issues, it would need a massive credit balance), so the fares complexity is all dealt with by the website or TVM issuing the 'ticket'.
That's not quite true - in fact GTR have a Pay-as-you-go system called
KeyGo which has now extended to cover most (but not all) of their network - i.e.
Southern,
Thameslink,
Great Northern and Gatwick Express. Worth noting however that journeys can only begin at station's with a yellow dot on
this map. Journeys can also be made using KeyGo on buses in eight or nine distinct zones, on the services on Metrobus or Brighton & Hove Buses. (The map and the website text are unclear on whether the Brighton zone covers Lewes, or Lewes is a distinct zone east of the Brighton zone.)
On the railway side, it can only be used on GTR services (and on a couple of stretches of GWR services in the 'Southern' area) - there's no ability to travel to London and then use the KeyGo on TfL services (e.g. the Tube or London buses) or even on any other NR services.
You need to register a payment card and actively opt-in to KeyGo in order to use it, though it is also possible to add credit to the KeyGo account via Payzone retailers (doesn't appear to be possible to top-up online). Fares are calculated the night afterwards and then billed either to the payment card or debited from the KeyGo account's credit balance.
There are currently no Railcard discounts available on KeyGo, which is a bit rubbish. Something else that's less than impressive is the requirement to
"allow two minutes between your exit tap at a train station and entry tap on the bus to ensure that your Key Smartcard is read correctly on the card reader" - no rapid intermodal hop off the train and straight onto the bus for KeyGo users.
I haven't checked in on KeyGo for a while - when it started it was just on offer for part of the Southern network and if I recall correctly it didn't initially include London destinations. It does now look somewhat more impressive, and does show that forms of smartcard PAYG are possible on NR - though definitely not on a national basis (rather, on a TOC basis).