I could imagine metro LA supplanting metro NYC if it developed with more density. Instead most development was single family housing, limiting how much its population could grow. In the 1950s LA considered building a monorail system (more info in the youtube video below), but it never happened. At the same time Toronto was building its Subway, which resulted in denser development.
In my opinion if LA had built a BART or Washington Metro like system in the mid-20th Century it could have realistically reached metro New York's population. Of course, that's assuming New York had a worse late 80s and 90s urban recovery/revitalization.
