SMPPS also never pays out more than the pool has. The reward system isn't the delay-- the payout system is. Generating all payouts means only 50 BTC can be paid at once, even if there's 51 BTC passing the minimum payout at once.
OK, I'd like to clarify my understanding of this a little bit. Let's assume that we have a fresh start, with everyone having zero balances. We find one block, so the total of everyone's pending balance is 50 BTC. Out of this 50 BTC, let's say 40 BTC is above the payout threshold, so they will be included in the next block. I assume the remaining 10 BTC goes into the pool buffer. Now 60 BTC is owed. But only 50 BTC can be paid out every block. I assume with SMPPS a few short rounds can clear out the pending payouts since the pool will owe less on those rounds than it earns. With PPLNS, I don't see how the pending payments can ever go down - since at every round the pool makes 50 BTC, owes 50 BTC, and pays out less than 50 BTC. This would mean that payouts keep getting delayed more and more?
Hmm, I think I understood it while typing this out. After some point, all 50 BTC earned in a block will be paid out because it meets the payout threshold. But the pool buffer that you've built up to this point is basically yours to keep, since it can never be withdrawn from anyway. Do I understand this right?