The problem with this is that nearly anyone could be reported for essentially any reason. The first job is separating the phony/disgruntled/revenge tips from the legit ones. Then you have to separate the real threats out of the pool of legit tips. An organization only has so many resources to devote to this kind of thing. And even if you follow up, many could probably slip through.
The point is that yeah, the FBI probably dropped the ball, so did the local authorities, but how can't they? It's a near-impossible task that follows terrorist math: "the murderers only need to be right once, the FBI needs to be right 100% of the time." Further, one wonders how many times they are successful in responding to these tips. There aren't any numbers on tragedies that never happened because of this system, and there never will be. But, obviously, the one crazy that gets through is never going to be tolerable for anyone.
Behind all of this, really, is the uncomfortbale acceptance that "better mental health" is a nonsense canard. It is only ever going to be discussed in hindsight, because none of this gets fixed until society as a whole puts on its big boy pants and starts making some hard decisions about changing this culture. That means actual sacrificies from people, for the common good: something we used to be pretty damn good at. Large sections of our population have lost their way, however. It's now all about "Me and muh Freedumb!"