LA is currently investing a lot into building more light rail infrastructure. However, regardless of how much LA invests in rail, for most commuters the rail will only get them most of the way to their workplace. Before UberX and Lyft, the only options for the last leg of the trip were bicycle (which has a lot of limits), bus (too unreliable), and taxi (too expensive).
UberX and Lyft provide an option that didn't exist before. And it's cheap and reliable enough to a be a very good option. As LA continues to build more rail, I think it has the potential to greatly change commuting in the future. Maybe it will become common for young single people to not own a car and rely on Uber/Lyft combined with rail. And then when they get married and have kids, then buying one car makes sense. The result would also greatly reduce traffic and air pollution.