Anything that doesn’t break even suffers from its own success.

If you have a public transportation system that loses money on every rider, then more people using it means everyone has to pay more (in taxes).

This can all work out when the economy is good and taxes can be increased, but it’s an inherently fragile system. At exactly the moment when most people will be dependent on a publicly funded system — when times are tough — is exactly the same time when tax revenues drop.

By creating a system that can’t sustain itself, you are making the system more likely to collapse in a crisis.