Wouldn't a simpler solution to this is to charge all of the cards a first year AF instead of the first year waived? This would eliminate churning for a lot of people but Amex could also throw in terms like must have paid the AF for 3-4 straight years before applying again 2-3 years later to receive the bonus.
I would have probably never applied for the PRG card if i knew i had to pay 175 up front the first year.