Why Not Build Both?

The LA Times has an article about planning for the Subway to the Sea. They've decided to not only study a route on Wilshire but on Santa Monica. When you look at the ridership on both lines it seems to make more sense to put it on Wilshire, and the property owners in neighborhoods along the line are stuck in the stone age. But here's a crazy proposition. Why not build both lines!? It seems to me that both routes would be good and will be needed in the future. It's interesting that people look at what ridership is now, because bus ridership doesn't tell you what the rail ridership is going to be, especially with the density and amount of young people in West Hollywood who would probably ride a train but not a bus. Another issue is the distance that people would have to travel from Union Station to get to the Sea if they used the Subway. But it would be shorter for people from the North. So really wouldn't it increase ridership on both if they built both. As Houston showed with its network effect, the sum is greater than the parts.