Note that in Primecoin difficulty increases exponentially.  If a difficulty of 8 is, say, 30 times the difficulty of 7, then a difficulty of 9 is 30 times the difficulty of 8.  To get to a block reward of 10 we would need a difficulty of 10, which is hundreds of times the difficulty we have today.  To get to 5 we would need a difficulty of 14, which would mean we were generating primes worthy of the current record books about every minute.  If we got to a difficulty of 17--the longest known Cunningham chain right now--we would still have about 3.5 XPM per block.  Rewards are certainly going down, but decreases in miners' profits will come more from increasing difficulty than from decreasing reward.