Actually, this past payout was the very best I've seen in a while. That is in the sense that the decrease from the previous week was the smallest we've had in quite a while. That ought to put things in perspective. We hope to see that happen more often, because eventually the payout will go down to the level of bitcoin dust.
What you say is true, but there is a logical reason for it.
Difficulty changes every 2016 blocks. In a perfect world, a block is discovered every 10 minutes, so difficulty changes every 14 days. If difficulty is rising at 10%, then a block is discovered roughly every 9 minutes. 2016 * 9 minutes = 18,144 minutes or 302 hours or difficulty increases 12.6 days.
A payout and difficulty change both happened right around August 31. The next difficulty change happened a few hours before the September 14th difficulty change. This would result in a very small decrease between the Sep 7 and Sep 14 payouts. You can expect the next payout to be about 8.5% less.
To put things in perspective, 18 months ago we measured our mining capacity in megahashes. About 10 months ago, gigahashes became the standard. Now you need to have some terahashes to get any serious income. Your payments will only turn to dust if you don't increase your mining capacity.