Surely the point that is not being expressed here is that on the BahnCard or similar - the calculation is that people will make journeys worth at or around €50 euro. That’s how every subscription model works - no-one at Netflix HQ values the entire content library at £8 a month - but it’s a price they feel they can get for the average that people might consume.
So the Commuter on the Ashford route is unlikely to pop to Wick every Tuesday to maximise use of their card, and therefore adding that journey to their subscription is irrelevant, since aside from a week or two of holidays where they will seek to maximise its use, it will be an unused bit of their subscription.
I do wonder given the forever promises of fares reform, and the Byzantine layers of complicated pricing and demand management which UK rail has suffered from for years, some variant of a subscription model which cuts through said layers may be simpler than trying to unpick the fares system, with all the winners and losers that would bring. Even if what is included in the package might largely be unused.
To cope with the regional differences, one wonders if two other ways of looking at this might be:
A Card which you get with season tickets over a given value (argue about that value) which gives you a carnet of “go anywhere” days inclusive, essentially following the rewards flight model of airlines, where people’s day to day travel is rewarded by a “free” holiday flight.
Some reform of the “family” group ticketing model, such as an extension of the “kids go free” scheme which might more challenge the comedy pricing for middle distance journeys which are usually much more affordable by car, so could probably be advertised as a green measure too. The issue here is of course we are using that pricing to manage demand on under resourced railways … mainly onto the M1, it seems.