Clearly there is merit in holding (supporting?) a Network Railcard for travel in the South East at present but when it is withdrawn, which it absolutely should be, it will just be one of those things and the Railway will be able to charge me a higher fare when I need or want to travel.
Lets take an example. The Anytime Day Return fare from Cambridge to London is £45.60. I would imagine that is pretty close to the economic cost of providing that train service. An off-peak day return is £27.60 and that becomes £19.50 if you go via the West Anglia line. At weekends, there are even cheaper fares at £18.80 via any route and £13.30 via the West Anglia line. This seems to have been set to be competitive without a railcard but a Network Railcard makes that latter fare £8.75.
It is preposterous that the Network Railcard is valid on that latter fare - it is effectively just discount (Network Railcard) on top of discount (Weekend) on top of discount (West Anglia route) on top of discount (Off-peak vs Peak). If the Contactless fare is set at £15 each way off peak and £25 each way peak, people will still travel and the railway will make more money.
Now, there might be some weeks in the year when the Railway notes that there are no events in the calendar likely to lead to people travelling and encourage use on those dates. That is when the railway should be offering discounts, not every week. [I note that there is nothing like Northern's off-season newspaper offer in the South of England - maybe that should happen on selected dates elsewhere - preferably on a day when it is raining and people don't all fill trains to the coast.]
Discounting needs to be about filling capacity that can't fill itself. A railcard for all, like the Network Railcard, simply isn't focussed enough in the current age.