Don't get me wrong, people do take the train in LA. The red line before the pandemic was packed full of people during my morning commute, transporting 130,000 people a day. Standing room only for the entire subway train. That being said, you'd never have people waiting on the mezzanines at these stations. The platform is already big enough to hold like 3-4 trains worth of people, and a train comes every 10 mins (every 5 mins where the line is staggered with another subway line downtown) and it is cleared out of people waiting when the train comes (sardines squeeze well).

To expand capacity beyond that would require reengineering the tunnels themselves. They go as fast as they can turn around and vent air as it is. You'd have to bore parallel tunnels to increase capacity, which would just mean adding more platforms and still wouldn't require a mezzanine.