Earlier quoted context omitted.
You're missing the point. There is feedback: usage stats. Public transport is already heavily subsidised in most European cities. One of the aim is to cut car use, not to specifically balance the books of just public transports. The issue is thus to avoid using taxpayers' money to finance lines that run mostly empty.
If people are driving in, is cost what’s stopping them? At least in the US public transport riders have lower incomes than those who do not.
This can improve passenger flow (I live in Paris, and turnstyles are a major pain during rush hour) and passenger convenience.
In the case of Paris, traffic is positively horrible, and parking is even worse of a joke. Yet there are people who still prefer driving and who don't even come from the suburbs (where public transit may be somewhat lacking). The reason is clearly one of convenience which means that this should be taken into account by the authorities if they really want people to drive less (which is the official policy here).