I don't think there is anyone suggesting that DFW's premier position in the Metroplex will be challenged.
However, DFW was the ONLY option for service to a number of cities and, since the DL hub shut down, a large number of the top 20 domestic markets from DFW have had only AA service.
thus, ANY service that is added by another airline and from another airport is going to create customer choice for Metroplex travelers and decrease the size of AA's share.
Perhaps, and it is far too early to know, but perhaps the market will grow to decrease the amount of share loss AA suffers. but since AA is not adding service, the only way their share won't decrease is if they rework their revenue mgmt. systems to take higher percentages of local traffic on their existing flights.
Further, WN knows full well that they cannot expect to pull much traffic over to DAL without reducing fares so the chances are very high that average fares from the Metroplex will fall.
Add in that some markets will have 2 new competitors with both WN and Virgin and the impact could be greater, perhaps offset somewhat by Virgin's departure from DFW.
The only real question is how well Virgin and Southwest will coexist at DAL but I would bet they do given their different product focus.
No one is expecting that DFW will be displaced as the largest or most significant airport in the Metroplex but there is no doubt that the growth of low fare carriers in N. Texas and the addition of alternatives to markets which have not had one in years, if ever, will rearrange the N. Texas market.