Thenyou must surely be in favour of a National railcard as that is what it would do !
A National Railcard would not be positive for net revenue, because the economics of supply and demand that made the Network Railcard work in the London area do not apply in the rest of the country. I'm not in favour of a National Railcard because I think it would result in a loss of net revenue for the railway, jeopardising its future, and that it would result in an increased financial burden for taxpayers.
The Network SouthEast railcard was a product of place and time.
On the supply side, London had a far more accentuated commuting peak than anywhere else in the UK. There were lots of resources, especially trains and traincrew, that were spare capacity off peak, either running round almost empty, or sitting idle in carriage sidings and mess rooms.
On the demand side, there was an existing market for day trips to London that wasn't using the railway. For example, when I was growing up in Cambridge, day trips to London were usually done by driving to Cockfosters or Stanmore then getting the Tube, not by going on the train. The Network Railcard captured that market.
The Network Railcard also grew that market, partly because it coincided with the relaxation of Sunday trading and pub opening rules. Day trips to London on a Sunday weren't a thing until the late 1980s because there was nowhere to go and nothing to do.
And it is a very big market, look at the ORR flows data. Of the top 10 flows with one station outside London, two are airports, two are Birmingham and Manchester, but the remainder are Network SouthEast stations to/from London: Reading, Milton Keynes, Watford, Woking, Cambridge and Chelmsford. Lots of that will be commuter travel, needing lots of peak time capacity, but lots of it will be travel using the Network railcard that would otherwise probably not be on the railway at all, travelling off peak on what would otherwise be nearly empty trains.
This discussion originated with the Network SouthEast boundary, and it neatly shows how the relative impact of these change with increased distance from London. Places like Ipswich, Peterborough, Rugby and Swindon don't have the same off peak spare capacity, or the same London day trip market. Bringing places like these into the railcard area would reduce revenue, not increase it.
A National Railcard would not work in the same way as the Netwrork Card because there isn't the same existing peak time capacity to move large numbers of new people off peak, and because there isn't a big equivalent of the "car plus Tube" market to capture.
Finally, I suspect that if the Network Railcard didn't exist, it wouldn't get invented now. By comparison with 21st century yield maximisation algorithms, the railcard as a marketing tool is a blunt instrument. Some TOCs already recognise this: for example Greater Anglia have offered advance tickets at very quiet times that are below the £13 railcard floor. The railway could get more revenue if it could use yield maximisation to vary prices according to demand. An example here is sports events, where a Network Railcard is a bargain for many supporters of teams in the railcard area, giving bigger discounts than would be the case with yield maximisation algorithms.
The railway does attract a lot of public funding, therefore its appropriate to run it on a public service, rather than an overly commercial basis.
The railway does not live in a vacuum and needs to reduce its public funding, not increase it. There are lots of other big demands for funds from other public services that are higher than the railways on the list of priorities for funding. Like any other public service, it needs to minimise its burden on the taxpayer by driving revenue up and costs down.
It isn't just about marketing, but marketing does have a significant part to play, because it helps to drive up revenue, and reduce the financial burden of the railway on taxpayers.