Re: How would we know?
Your argument could apply to rare but very damaging events like asteroid collisions or supervolcano eruptions. However, the likelihood of Earth being destroyed via the LHC producing strangelets or black holes is zero. The Earth is bombarded every day by cosmic rays accelerated by supernovae, neutron stars, galactic mass black holes, etc. to energies far greater than any the LHC can hope to achieve in the foreseeable future, and this bombardment has gone on for the last 4.5 billion years. Furthermore the Sun has an even greater surface area and undergoes the same bombardment, and it's still here too. And every star in the sky undergoes the same bombardment and we don't see them suddenly disappearing. Caution is one thing, ignorance and the lack of any common sense or rational thought is quite another.