The new point you added about what will happen when the block reward goes from 25 to 12.5 is very interesting and I hadn't thought about that previously, but it bears consideration especially if you bring it to its logical conclusion: a share becomes worthless when the block reward becomes 0. The payout logic remains the same, (block reward + transaction fees) worth of shares are paid out, but the intrinsic "value" of the share, which previously was always some positive value in BTC, becomes zero.
I have a question for you.
If you're 5 feet from a wall, and each step you take, you cover half the distance, how many steps will it take before you're there?
(the block reward will never reach 0)
M
The ideal mathematical case is that it takes infinitely many steps, so you never reach 0, but Bitcoin cannot be subdivided smaller than 0.00000001 BTC. In 2140, the reward will round down to 0 and no new bitcoins will be created. However, that is very far in the futures, and many things will change from here. (One probable thing that might happen is that the protocol might be modified to make bitcoin divisible to smaller unit)
Anyway, the point is moot, as it is know that the current algorythm of Eligius is unsustainable in the long term. It is built as if transaction fees do not exist; and indeed at first it didn't even pay them to the miners. The fact that the transaction fees are put toward paying the share log is a relatively new thing (a few months), but the shares are still 'created' as if the the transactions fees were not paid to the miners.
That problem is not really significant currently, because the transaction fees are insignificant, but I'm pretty sure the pool operator will address this when the transaction fees will become significant. It is just difficult to devise a "fair" way of paying the transaction fees to the miners, because contrarily to the block reward, their value is unpredictable and varies a lot.
Your last sentence is exactly what I was driving towards. While it may be a long ways off and chances are pretty good none of us on this board will be around to see it, the transaction fees are what will be utilized to incentivize miners of the future. A lot of people, when discussing the block reward halving in the next couple years fall back on the argument of, "Yeah, but BTC will be worth double what it is now, so it all balances out." What would be really nice to see is the widespread adoption of BTC globally, and as such transaction fees playing a larger role in payouts as well. As you very correctly point out, Eligius in its current form does nothing with transaction fees except to pay off some more of the shelved shares.