No worries, says Braves fan, author, and economist J.C. Bradbury:
I consider major and minor league sports to be complements not substitutes. While I am sure there will be some families that will choose to attend more Gwinnett games at the expense of Braves games, I think this loss will be minimal. The opportunity cost of going to a Gwinnett Braves game is most likely doing something at home, not going to a Braves game. The proposed stadium site is nearly 40 miles from Turner Field. My guess is that most fans will switch from doing something else in Gwinnett rather than canceling trips to Turner Field; thus, this will be a net gain to the big club.
Click through to read the basis for J.C.'s conclusions. He has convinced me.