It's tough to set any simple firm rules that maintain the right incentives, because (roughly) each byte incurs costs on every node storing/verifying the blockch

It's tough to set any simple firm rules that maintain the right incentives, because (roughly) each byte incurs costs on every node storing/verifying the blockchain, while any set one-time fee can only be collected by some subset of those nodes up-front. Thus you can expect continuing push-and-pull negotiations, perhaps requiring a separate system of continued payments, if you really want indefinite, reliable, unalterable storage.