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.
Merchant are also giving a percentage of their margin profits when customers pay fees. Demurrage fees are part of the merchant costs and customer will pay them indirectly just like they pay for bank fees.
And demurrage can be much cheaper then those.
It is possible that users prefer tx fees, though. Specially if they're not aware how much they will
save thanks demurrage.
Most people think that they don't pay interest if they don't borrow money, but that's not true. Interest is factored in the price of every item you purchase.