Isolated event, and the guy was brought down. There'll always be a risk as long as their are fanatics or loonies who don't give any though to their own personal safety, but there comes a point of diminishing returns.
Suppose they hired 10 times as many Secret Service agents? That just increases the odds of one of them going bad and offing the President himself. (Not a likely event, but having 10x as many agents also means more chances of confusion in a crisis, etc, etc.)
Security is never perfect (wasn't there an incident some years back where an intruder wandered into the Queen's living quarters at Buckingham Palace?) That's one reason we have a line of succession -- it's not like the government collapses in the case of an untimely death.
Mind, given the choices of VP over the past few presidencies, that line of succession might actually be helping lower the odds of someone trying to assassinate the Prez.