If we are going to have a valid thought experiment ... if SoCal fielded a National Team, would it beat the US (minus SoCal national team), then we have to look at this slightly differently.
SoCal is 25 million souls. The USA minus SoCal is about 305 million souls. So purely on a population basis, we should find more "unicorns" in the 305 million population. However, we know that many of the players in the 305 million population will never have an opportunity to fully develop into a unicorn because population density in their states make it near impossible (Alaska, Montana, Main, etc.).
Rather than weeding though what States and population densities are adequate, let's make it easy and ask is 25 million enough to find 24 unicorns (i.e. elite players that can make a national team) ... and if we are being honest we really only need 15 to 16 that will actually play and get minutes.
With a population of 25 million, are there 24 "unicorns" to be found in SoCal? We can infer that countries that are perennial competitors at the Women's World Cup will give us the "floor" to the question. Therefore, all we need to do is look at countries like England (66 Million), Norway (5.3 Million) , Germany (83 Million), Sweden (10 Million) and Japan (126 Million), which over the last few World Cup cycles are perennial top teams.
We have floor ... Norway with a population of 5.3 million - Champions in 1995, runner up in 1991, 4th place in 2007 and 1999. In fact, Norway is ranked 3rd behind German and the US as most final 4 finishes in the Women's world cup.
Is is reasonable to conclude that SoCal could field a team that would beat a similarly aged girls/women's youth team from any other country, including the US? Yes, because Norway tells us that 5.3 Million people is enough. But, its also reasonable to conclude that the SoCal National Team would get beaten more frequently than it wins because there are more unicorns to be found in the 303 million population.