I still think that you can keep mining sustainable with demurrage, maybe even only with voluntary fees.
Merchants will not give up even the smallest percentage of their profit margin unless they have to. A mandatory TX fee is the only way to extract blood from that particular stone.
I think demurrage could work, but that users would prefer txn fees. Simply because people won't like the idea of money they have forgotten about slowly being drained away.
It is like signing up for a credit card with annual fees. I would much prefer that they gouge me only when I purchase something, rather then when I am not paying attention.