Drop rate and usefullness impact the market. Right now, if I tried, I could maybe buy a medium priced Promo with some mild trading and daily playing within a week - free BFP every day for anything you do...
Yes, I suggested Promo drops be halted completely after a certain point, so they actually stay rare. New players will come, but the slower the influx, the less rare all cards are (because thats how maths and logic work). With a limited cirtulation, their prices can only go up, making them a worthy goal to strive towards getting, while now it's either saving up for a bit or just getting lucky, both of which are quite boring.