To clarify....assuming lots of people decide to use A3 for status only, and some other program for accruing all miles after the first 20k. Then A3 are likely to move quickly to change the program as the costs would be very high unless they also get the benefit of being paid when members credit miles.
This is against Terms and Conditions, and a contact of mine at A3 said that if this gets out of hand, they will start deleting accounts which violate this policy.