In real life ATL isn't slot restricted at all... Neither is ORD... Slot restrictions in the USA are extremely rare - only four out of hundreds of U.S. airports are subject to federal slot restrictions: LGA, DCA, JFK, and EWR. ORD was slot restricted before it opened the new runway in 2008. All other airports are actually forbidden to artificially restrict flight operations through slot systems, etc. but a few community airports are exempt since their operating procedures were established well before the 90's and thus "grandfathered in". So, even at busy airports like ATL, ORD, DFW, DEN, LAX, IAD, FLL, BOS, SEA, MIA, PHL, SFO etc. there are no slot restrictions in place that would make incumbent airline expansion and new carrier market entry inherently difficult. So long as airlines have the terminal space/gates to support their ops, flights can be readily added and dropped at any U.S. airport without a problem.