Instead the legacy had to cut wages and abandon routes because there are carriers like VX willing to throw seats in a market at ridiculous cost to attempt to gain market share.
From this article, it appears that VX is working at raising yields, at least to the level of AA. I highly doubt that a 39 ac domestic operation would drive AA's yields down across the board. Seems to be a pretty detailed and unbiased article covering both aspects of the costs and revenues of both legacy and lcc carriers.
Low-cost carriers: growth expectations
...So as low-cost carriers add product, and with it some cost, and network carriers strip out frills and costs on short-haul, the line between the models blurs further. Almost everyone can lay claim to being a hybrid.
Oliver Wyman's recent Airline Economic Analysis shows the cost per available seat kilometre between network and what it calls value carriers in the USA has narrowed to its smallest since the study began seven years ago. And there is a similar blurring on the revenue side. "One of the things that really stood out in the study is the US unit revenue - American and Virgin America are virtually identical," says Pomeroy. He notes that JetBlue is not much further behind.
"I view it as converging in all directions," he adds. "For example, American has lots of ancillaries, it has added 12 more seats [in the aircraft], while at the other end value carriers are selling through the GDS and have a frequent flyer programme."... (my emphasis added).