Cash is just the physical representation of a fiat currency. Fiat currencies exist independently of cash. As long as you can pay your taxes somehow with cash,

Cash is just the physical representation of a fiat currency. Fiat currencies exist independently of cash. As long as you can pay your taxes somehow with cash, I fail to see the problem? The tax office doesn't need to accept cash directly. I don't really see the problem if they delegate handling cash to banks or other institutions as long as no one can be denied access to those.