EarlyBird will cannibalize BS to some extent, but remember: EarlyBird is not available less than 36 hours before departure. I suspect that most BS purchases are made within 36 hours of departure. Those late BS purchases will not be lost.
According to
http://www.southwest.com/flight/early-bird-faq.html
While the best EarlyBird Check-in boarding positions will be assigned 36 hours prior to the flight's scheduled departure time, Customers are eligible to purchase EarlyBird Check-in up until 25 hours prior to the flight's scheduled departure time.