From my mathematics training, I tend to speculate about changing rules, and what outcomes are likely.
This weekend, I wondered what would happen if the "recent backpay" was changed to "oldest backpay."
One effect is that the very old backpay would be paid out. In the current system, there is some doubt whether it will ever be paid.
Another effect is that new miners would see a drop in their revenue, because initially they would not receive any backpay.
Possibly a blended system, 50% recent and 50% oldest would balance these two concerns.
I assume wizkid & Luke evaluated this question in the past. I wonder what their reasoning was.
They have indeed evaluated this. Eligius used to use SMPPS which is very similar to the current system but with oldest backpay. If I understand it right, the only difference with SMPPS was instead of the logic with "shelving shares" it paid out every share but upon bad luck reduced the amount paid per share proportionally. Upon good luck, these underpaid shares got paid back "extra credit" (in the order of oldest first) until every share gets paid the full PPS amount.
This system sounds OK in theory but the problem is that luck does not even out to 100%. According to Eligius's stats, the luck for the entire duration of the pool evens out to about 98%. This is due in a big part to orphaned blocks (and maybe a small part due to some miners withholding blocks.) In any case, this means if you backpay the oldest shares first over time the backlog will grow faster than the pool can pay it off. Eventually the pool will end up in a situation where the backlog reaches a very undesirable length for new miners. Say it gets to the point where it takes 3 months to pay back everything in the backlog. If you're a new miner this means you won't see any benefit from lucky blocks for 3 whole months until your first shelved shares reach the front of the queue. Also the people mining 3 months ago may no longer be contributing to the pool (and if it's been that long who knows if they even still control the payout address.) A lot of people thought it unfair that current miners were using their hashpower to pay people no longer contributing. The current miners are also depending on the pool still mining 3 months down the road in order to receive any of their backpay. (if the backlog gets that bad it's likely the miners will abandon or the pool with switch payment methods like Eligius did.)
Another way to think about it is that oldest backpay is unfair because it benefits the people who mine on the pool when it first opens (and eventually punishes new miners as the backlog grows.) This is because there is no backlog initially so chances are good that all your shares will be paid out 100% making it effectively a 0% fee straight PPS pool (with your backpays being only slightly delayed.) This low/no backlog state can also occur later if the pool gets some very good luck (likely while the pool is still relatively new since, given time, the backlog will inevitably grow faster than it can be paid off) and this encourages pool hopping. If someone sees the pool has a low/no backlog they can benefit themselves by hopping on it while the chances are good that they'll get all their shares paid 100% in a timely matter.
In any case, when a pool with oldest backpay shuts down or changes payment systems (which will likely happen to any such pool when the backlog inevitably reaches unacceptable levels) the end result will be the same: the original miners and the pool hoppers who mined when the backlog was low and then bailed when the backlog grew will get all their shares paid 100% while the new miners may never get any of their backpay.