My understanding is that the legal argument is going to revolve around whether woman's soccer is "equal work" to men's soccer.
That is the only response USSF has. I can't see them successfully arguing any other position
1) The market value of a person is clearly relevant to their salary. Tom Brady earns more than some random linesmen or even other quarterbacks. People get paid according to their market value.
This is not true for the national team. USSF has a complete monopoly on participation on the national team. All players get the same salary (different for men than women). The law in question here says that you cannot discriminate based on gender among other criteria for people doing the same job so I cannot hire male welders at one salary and female welders at another if they are doing the same job. This is especially true if I am the only employer of welders in the area. The USSF will argue that if you look at the world in general, male soccer players earn more than women so they are justified in paying less even if they do the same work and even if that disparity is due to its lack of promotion. The law says in the US you can't do that.
2) The racial issue in sports was solved by combining racial leagues to create one single professional league (there isn't a Black NFL or a White NFL), it is just the NFL and all the players compete for salary on mostly equal terms. Unless you think we can get rid of the woman's league and have everyone complete on equal terms, this isn't a good comparison.
You make that sound so egalitarian. The sports leagues were dragged kicking and screaming into desegregation and often it boiled down to money and legal action (like the one happening here). I mentioned women's tennis which is the only place in major pro sports where women approach men in salary. In 1970, after boycotting the Pacific Southwest Championships (now called Indian Wells) due to lack of a even half way decent women's purse, the women were BANNED FROM PLAYING by the United States Lawn Tennis Association (USLTA). That went on for two years before the USLTA cracked because the women actually did bring in equal money, attendance and press as the men. For soccer, the women do more work (play more games, attend more pressers and clinics, travel more) and bring in more revenue. They are treated in an inferior manner, not just in pay, but in accommodations, work conditions and support.
3) Soccer players get paid based on physical ability; how fast they can run, how hard they can kick, how high they can jump. Women do not have the same level of physical ability that men do. The comparisons of how women succeed playing against other women are really irrelevant. Men's teams are better (i.e. head to head they would win handily).
That is just completely wrong. Athletes are not paid by their times in the 40, they are paid by their performance in the game they are playing. Hope Solo is acclaimed for stopping shots on goal, Alex Morgan is acclaimed for scoring them. No one asks or cares about how fast they run the 40 or how much they can bench press. The same is true for the guys. By your argument, Floyd Mayweather, considered by many to be the greatest boxer of all time, should get a small fraction of the pay a journeyman heavyweight gets. I mean at 150lbs, no one is saying Mayweather could compete well against heavyweights, right? So what if his boxing skills against the little guys was insane and he could haul in the pay per view market, they were little guys, right? The reality for the US National Teams is that men field a truly mediocre product, bring in mediocre revenue and get paid substantially better than the women who are one of the most successful US sports team in US history and bring in substantial money, publicity and influence to the USSF.
I think the political case is going to damage them.
That is the very exact argument made against the women tennis players in 1971, except it was made much more forcefully with economic penalties and social condemnation. It turns out the women made a good call. The USWNT made some of these points in 2016 when this complaint was first filed and they won the political case in spades so I think they are on solid ground here. (If you read back in this thread, even you agreed.) There are lots of parents of teen girl soccer players out there talking about this. If any of them disagree with the national team, I'll let you know after tonight's game.
I think the economic reality makes this impossible.
Again, see women's tennis. The women already bring in enough to pay for their salaries. If the USSF doesn't agree, all they have to do it agree to the revenue sharing proposal the women's union offered. That should completely answer your concern, right? If the women are paid strictly on what they bring in, no one can complain. Of course the USSF turned that down because they count on the revenue from the women to subsidize the men.