Any number of cities have passed laws/ordinances/etc that prohibited commercial service from their "old" airport to protect the investment in their "new" airport. The catch that WN hooked onto was that when they first started flying, they flew only DAL-HOU and back. Therefore, they were not covered by Federal law other than FAA regulations which apply to all carriers regardless of route.
The Wright Amendment was passed by Congress either when (or in anticipation of) WN announced the plans to fly outside of Texas (therefore making interstate commerce laws applicable to them); and for the same reason as the original agreements between Dallas and Ft. Worth...to protect the HUGE investment (local and Federal) in DFW. It restricted WN to nonstop flights from DAL only to the 7 contiguous states to Texas. Thus, the humongous increase in the flights from HOU to other places as WN grew. (WN also "forced" the reopening of HOU to commercial service. Houston had a similar restriction on traffic out of HOU once IAH opened. But, at the time Houston's growth was a net increase of 1 person every 14 seconds, 24 hours/day, 7 days/week; so, it was hard to argue that the city couldn't support 2 airports.)
The courts have on more than one occasion ruled that the Wright Amendment is constitutional--including the amendments to the Amendment (no pun intended) that have occurred over the years resulting in DAL nonstop service to cities outside the original perimeter.
Aside from the gate shortage at DAL, the airport itself is located almost downtown in Dallas (less than 5 miles from city center), and regardless of direction, take-offs and landings are over heavily populated residential areas--including some of the priciest real estate in the entire U.S. The residents of these areas are not happy about the number of daily flights now. No telling what's going to happen in the near future. I see court actions coming--noise abatement, basic safety involving low flying a/c, etc. I'm not psychic like WT, but I would not be surprised if DAL ends up being a slot-restricted airport like LGA and DCA.
If Dallas had done as Denver did--i.e., demolish the old airport terminal and dig up the runways--we wouldn't be having this conversation today. And, if Denver had done as Dallas did, WN would be flying to the old airport and their customers wouldn't have to be driving to western Kansas like the customers of the rest of us.