There are more miles than seats out there. Artificially capping the total number at 37,500 makes no sense.

The second point I was making was to counter your theory that having one-way rewards would preclude return trip folks from booking rewards. They would still book them, more easily than before because they can pay for the return trip in cash and not have to find both directions as a reward trip. Sure, that means the airline gets some cash and some miles off the books but most of the time they prefer all cash and no miles redeemed.