PCGS reports just over 5,100 1907 HR's and NGC just over 3,800. So about 8,900, making for just under 3,500 coins not accounted for.
If you assume lots of crosses from PCGS to NGC and vice-versa (double-counting), then maybe 4,000 or 5,000 coins not accounted for. Could be more....maybe 6,000 or more.
If the "unaccounteds" remain in private, "sticky" hands -- handed down by from family members to heirs -- that could account for the sticky and relatively high price of 1907 HR Saints. If a few hundred 1907 HR's were to hit the TPGs and market every year for the next decade or so and everyone knew they'd be coming out (with grades across the spectrum), my guess is you'd have a slow, steady drizzle downward in price.
But I'm not sure how much.