This is just a tax on savings. The average early adopter is going to be strongly against this as well they should be). You are suggesting that we impose an artificial storage cost in order to make bitcoins more like a physical commodity as if that were a worthwhile goal. We don't want "digital gold"--we want a good medium of exchange. Why on earth would holders of BTC want to pay to store them when they cost (virtually) nothing to store?
Long term storage of capital is not free for the network, even though it might seem that way. The network does suffer an uncompensated cost. Namely, the ongoing replication of those deep transactions as new clients bootstrap and the ongoing disk storage costs, however small those might be individually, multiplied by the number of nodes. What I'm suggesting is an incentive for capital accumulators to consolidate their holdings into fewer/newer transactions, allowing the network to 'compact' the blockchain. Currently there is no incentive for early adopters, or anyone else with any substantial holdings, to spend their oldest transactions first. This is the default action of the client, and this might be enough, but sooner or later someone is going to mod the client to allow users to spend newer coins first, because the deeper the transactions are the more secure they are. There does need to be a cost for that kind of long term security, particularly if those holdings are spread across numerous transactions that cannot be pruned. I do like the proposal of a miners' choice and rear-loading the fees. It permits the well heeled bitcoiner to contribute to the security of the blockchain in a less direct manner, for the only miner that is likely to accept his old transaction without the demurrage fee is one that he owns or is otherwise closely associated with anyway. If he is owner of a bitcoin bank, his old transactions can be spend or freshened without fee only if his own bank is doing the processing; implying that his financial actions directly benefit the security of the blockchain because that would have to be true for his transaction to be accepted for free in any reasonable period of time.
I have a proposal. Using the rear-loaded, miners' choice model. A minimum fee rule for any new transaction with inputs that are older than a year (in blocks) will have an
alternative minimum fee based upon demurrage of one Satoshi ( .00000001 BTC) per retarget cycle (2016 blocks) for every input that exceeds one year since it's transaction was recorded. This means that each
input is charged for demurrage from it's inception, not the end of the first year, and the minimum fee for a new transaction with a single transaction exactly a year old would be at least .00000026 BTC. This isn't much at all, but would still incentivise some savers to either freshen their savings once each year, consolidating their many transactions down to one while doing so, and
potentially paying a transaction fee for the effort; or resolve to pay for the network storage costs upon release of funds. This even gets demurrage upon off-network transactions wherein the private keys are traded instead, because no one saves money to never spend it, so sooner or later that has to happen.
This rule need not go into effect until the block reward is cut to 25 coins, giving the early adopters plenty of time to plan out their best course of action, most of whom will be consolidating numerous 50 BTC transactions into a single (bytewise small but BTC-wise large) transaction; permitting the network to prune even the block reward transactions from the old blocks, perhaps all the way down to the headers alone.
Also, this rule would be an alternative minimum fee, so if some other rule required a higher mimimum fee, those fees would not be additive. It's just whichever minimum fee is highest that is required; or a miner willing to process your transaction for free.
Alright, I'm ready. Tell me what you think, but please leave my mother out of it.