Ask anyone who knows the insurance business, and they'll tell you that rates get better as the number of policy holders increases. It's not about risk, but rather about volatility. With more customers, the numbers become more consistent year by year which lets the insurer cut margins closer without fear of being hit with a big claim and not being able to pay it.
That said, there's no way the AMA will lower membership dues if the insurance does become cheaper. They will just use that money to start a new program, raise salaries or create new positions, or increase advertising or flying site assistance.