The only viable long-term solution I see for most of these things is charging per-transaction for messages. Remailers are exactly the kind of application where "postage" could first be applied. The problems seem twofold:
I agree. Until remailers start charging per message, they will never be "sustainable". Yes, they may hang around but they'll always be in a position where a) the operator pays costs of operations and b) the operator exposes himself to various risks and liabilities and c) the operator isn't getting compensated for a) and b), which makes the whole thing inherently unstable or unsustainable. Charging for service is the only way to go.
2) It's much easier to trace payment instruments than remailer messages. While a blinded cash system might only leak "this person is using the remailer payment system", and not provide linkability to given messages, it does complicate things substantially, and provides the potential for a lot of tracing.
Yes, there are no "sustainable" non-tracable remote payment systems in existence, and it's almost impossible for such a system to exist. Why? There are many reasons. One of the most obvious ones is that it would be so disruptive to the activities of so many very powerful institutions (tax collectors and law enforcement mainly) that they will shut it down, where ever it may be, as soon as it gets big enough to be on their radar. Staying small enough to be under the radar is also not a sustainable strategy. The more fundamental reason why an untracable payment system is unsustainable is that it will be used to pay for things which society (including all of us on the c'punks list probably) abhors, such as non-state-sanctioned murder-for-hire, and eventually the operator will be found and put out of business one way or another. Is there no hope for remailer operators? Probably not. Note that I'm not trying to claim that remailers are impossible; they are possible and they have existed for years. They will just always be little personal projects, which will come and go. The first time an operator faces serious jailtime for running a remailer, probably most or all of them will shut down. This time may be coming soon.