Surely it would simply be better if every ticket inspection required verification of the possession of a valid railcard, either on the train or at ticket barriers. In the latter case a railcard would need to be presented before/after the ticket before the gates would operate (or just force a manual inspection as is done today already). In the former, well, I just don't know why on-train inspections often simply don't bother with railcards anyway.
It wouldn't stop barrier-less inspection-less fraud, but even at my barrier-less local station, it's very likely that my destination will have barriers.
Yes, it would slow down the use of barriers, but to be honest they are often clogged by clueless people anyway standing in the way whilst trying to find their ticket.
Given that railcard fraud seems rife, it would be a small price to pay to ensure that the majority of it gets stopped.
I'm not the target audience, of course, but I usually present my railcard unasked when on a train, and wouldn't object to doing so at a barrier.