Consider the following possibilities:
1. Nuclear War
2. Genetically engineered viruses
3. Natural (nonengineered) viruses
4. Nanotechnology robots which "eat" everything and self-replicate
5. Intelligent, larger robots that take over the world
6. An unanticipated, technological threat, developed in the coming decades, perhaps based on new discoveries in physics.
Can we stop any of these things? If nuclear weapons are any guide, then the destructiveness of the technology will be available long before a solution to the destructiveness is available. What happens when technology allows a small group of scientists, hell, a single nonscientist, to destroy the world?
You're right that the genie is out of the bottle, but he's one of those sneaky genies who twists your words around when you make wishes.
I don't know if we can stop those things, but I look at what we have stopped and I'm encouraged. A fifty-year standoff between the U.S. and the Soviet Union with enough nuclear power to blow up the earth several times over. Many killer chemicals for weapons have been known for much longer than that, and other than people like Saddam Hussein, not widely used since WWI. Many diseases eradicated, many vaccines, many new medicines to treat old diseases. I'm not morose by nature, so I don't sit around worrying about all the potential ways to die. Besides, most people are going to die of heart disease in their Lazy-boys.
If humans don't destroy it first, of course, according to astronomers the earth will be hit within a few million years by a giant meteor. Failing that, the sun will go through it's natural changes and kill all life here. But, as one astronomer put it, so what? It's not as though the earth is a major planet anyway. :)