'Dumb-belling' fraud maybe an issue, but I think it's overstated.
What evidence do you have to support that view?
IIRC▸ , whenever there has been a block (ie preventing passengers from entering a station close to a main station without a ticket), there has been a good hit rate of passengers arriving at the main station who claim to have got on at the aforementioned station.
Self service ticket machines need to be as simple as possible to use, with as few steps as possible. I don't think it's reasonable to expect
TOCs▸ to change their machines to help those trying (legitimatically) to get a cheaper fare than the quoted fare from their departure station.