Exactly, if people want to commit suicide they are going to do it. Putting saftey barriers up on an isolated bridge isn't going to stop them, and it's jsut going to annoy everyone else.
People are often quite quick to blame the 'system' whenever someone commits suicide, or whenever someone kills other people. Although undoubtedly the system needs more funds and isn't optimal - we have to put responsiblity elsewhere too. The roots of suicidal feelings for instance, the family support, the responsiblities of the community.
We can't save everyone, and prevent everyone from mental illness - as much as some people would like to believe putting more money into the systme would do. Even if you put infinite amounts of money in, I think you would still have people killing other people and themselves, and escaping the system. It's sad, but probably true.
As someone who has experienced both mental illness and suicide close to me, I feel the system has worked pretty well. That's no proof, of course.