Provision of card payment would require significant investment, not just in new gates but also in fixed wiring, as I doubt that the toilets currently have the necessary telecom connections. To be honest, I think you are lucky to have toilets on Sheffield concourse in the first place. Apart from the London terminii, most stations with barriers only have toilets on the platforms. If there are free toilets on the platforms for staff and bona-fide passengers to use, I don't see why there needs to be any toilets on the concourse in the first place. A lot simpler and cheaper to just close them than to provide card payment.