An off peak day return from Bath Spa to Cardiff Central with my railcard is currently £13.05. The off peak day single is £13. If you got rid of the return and I had to buy two singles that would then be £26, pretty much doubling the cost. Isn't that obvious?
Why are you assuming that the single would stay the same price? That has not been the case with Oyster, which is why people have been pointing out that the new Gatwick Express single fare is an anomaly. For your example, I would expect the single to change to between £6.60 and £7 to keep it revenue neutral. Indeed, my beef is particularly with fares like you quote, where the single is virtually the same price as the return.