This question can be rephrased like this: how can shooting myself in the head be bad for me in a world where shooting myself in the head is impossible to be bad for me? We could cure death and people will still say shooting yourself in the head is the deadliest thing ever.

So therefore, trying to prevent people shooting themselves in the head is not about health.

If nothing can kill you (even shooting yourself), then can anything (including shooting yourself) be dangerous?

I present this puzzle to others, they do the same thing.

Think of it this way, if we had death curing nanobots, does this mean you can survive a nuclear explosion? Will you become immortal?

The nanobots the book was talking about are nanobots that stop the tendency for the body to decay as we age. They are not meant to make us immortal.

Think of them like very advanced prosthetics.

Having the ability for prosthetic arms and legs does not mean that arms and legs (both natural and prosthetics) is impossible to be chopped off. Having heart pacers does not mean that poor diet and low exercise is impossible to cause heart disease.

People with poor health need more medical intervention and earlier than people with good health. The existence of inventions does not change risk factors.

We have a real life example. Oxygen is a very reactive molecule, it is unstable in large amounts and corrodes everything else, yet we need it to survive. Something that is bad for other things is required by us. The reverse is also true. CO2 is harmful for us but good for plants.

Hope that actually clears up your thought process and helps you stop believing in none sense.