More and more places are now offering "cash" discounts, I have had anything up to 5% discount at hotels, restaurants, garages, etc, plus a lot of places now add a "card surcharge" of, again up to 5%, don't think "cashback" is anywhere near that figure.
Cash is expensive to handle for any retailer, so there are also retailers that are willing to offer small discounts for card payments. I don't think it is particularly helpful to go down this avenue as such discount-by-payment-method is unlikely to happen with the purchase of railway tickets imo.
I agree that it is insensitive to claim that certain groups of the population are hardly worth worrying about, however I cannot agree with the notion that it is discriminatory if an additional payment mechanism does not cater for everyone's needs. Are Northern card-only machines discriminatory against people who only have cash with them? Are onboard machines, should they be developed one day, cater for people wishing to pay by RTVs? People with certain valid payment methods have always received differing treatment, such as RTVs and cheques not being accepted at TVMs. If the introduction of these new payment methods do not result in worsening treatment of passengers whose preferred payment method is not catered for by these machines, then I cannot see how this is discriminatory.
In addition, there will be other factors to be considered with the introduction of these machines, such as the security of these machines if they contained cash. I fear that in current conditions, onboard cash ticket machines are nothing but a pipe dream.
I think we are sidetracking the real issue here, which is the inadequate staffing by FGW along the Severn Beach line and at Temple Meads station.