The thing with adding lanes is induced demand. By nature of there being more space for cars on that road more drivers will choose take that road over other roads. Cars don't magically come into existence, people drive them, and people drive them for a reason, most commonly to go to/from somewhere

Trains (and bikes and buses) take cars off the road. Every person riding on a transit solution that isn't a car is a individual vehicle trip saved. When every vehicle contains an average of 1.2 people in it, you've got very close to 1:1 vehicle reduction for every trip that's not taken by car

So to your point, are some number of non-drivers choosing not to drive because of traffic? Probably a small number of them. But a complete transit system that has the real world effect of fewer cars on the road will mean few people owning cars. Why would a family own 2 cars when one is parked most of the time? Why spend $20k on a new (to you) car if you're barely using the one you have/had? Fewer cars means less cars on the road which means less traffic. This is the dream.