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In the long run, I think we have to assume that miners will do what is most profitable, and design escrow/payment protocols around that assumption.
Replacing non-final transactions in the memory pool potentially opens up a whole bucket of worms, but I think the short-term challenge is to figure out if we should change the rules we have now. If we do (I think we should), then I think it would be a mistake to do anything other than "create rules that will maximize miner profits." Because if we do something else, then sooner or later I believe there will be a "miners special" version of the code that has miner-friendly rules.
If you agree with me that the default rules in the client should maximize miners' profits, then it seems to me there's one simple rule change we should make:
If you've got two otherwise equivalent transactions that spend the same input(s), keep the one with the most fees.
Right now, the rule is "keep the first one you see."
The bag of worms comes into play if you've got two transactions that spend the same input(s) that, for example, look like this:
Transaction 1: final transaction (can go into a block RIGHT NOW) that has a fee of 0.005 BTC
Transaction 2: transaction that won't be final for 3 days that has a fee of 0.1 BTC
Should a miner put Transaction 1 into the block they're mining and take the smaller fee now, or not include it, hoping that nobody else mines Transaction 1 in the next 3 days so maybe they can mine Transaction 2 and get the bigger fee?
I'm not an expert in game theory, but I believe the winning strategy in the above situation, assuming everybody knows about both transactions, is to mine Transaction 1 right away (any economists reading who know a lot more about game theory than I do?).
That suggests the rules for transactions that spend the same inputs aught to be:
1. If you have two free transactions, keep the first one you see.
2. If you have a free and a fee-paying transaction, always keep the fee-paying one.
3. If you have two final, fee-paying transactions, keep/mine/relay the one with the higher fee.
4. If one or both of the transactions is non-final, keep/relay the one that will become final first.