It’s impossible to create an engineered pathogen that kills everyone

The main reason why scientists and policymakers are skeptical of existential biorisk is that it is terribly hard to engineer a pandemic that kills everyone.

First, you would need to reach everyone. The virus would have to be transmitted to the most rural and isolated corners of the earth; to antarctic research stations; to ships at sea, including nuclear submarines on uncharted, long-term and isolated paths; to doomsday preppers in their bunkers; to hermits and uncontacted tribes; to astronauts in space; to each new child born every second; to island nations; and so on. And you would need a transmission mechanism that could spread the virus this far without being detected: otherwise, those with the means would be whisked away to safety and might well survive.

Second, you would need a virus that was virtually undetectable until all, or nearly all humans had been infected. That conflicts in a stark way with the goal of producing a virus that is 100% lethal, since lethal viruses tend to leave a trace as they spread through a population.

Third, you would need a virus that is unprecedentedly infectious. The virus would need to be capable of being transmitted, without fail, to every human being on the planet, in sufficient quantities to actually make them sick. It would need to avoid respirators and other forms of protective equipment. And it would have to maintain its transmissibility throughout many generations of mutation.

Fourth, you would need a virus that is unprecedentedly lethal, killing not 90%, 99%, or 99.999% but effectively all of those infected by it, no matter their age, health, or genetic makeup. This lethality would need to be preserved even against the best medical treatments, including quite possibly vaccines or synthesized antibodies. And it would have to be maintained throughout many generations of mutation despite selective pressures towards less lethal variants.

Fifth, you would need to find a way to evade basic public health measures such as masking and social distancing. This isn’t as easy as it sounds. How do you transmit a virus to someone who doesn’t leave their house?

Sixth, you would need the technological capability and equipment to synthesize the hypothesized biological agent, something it is widely agreed that humanity currently lacks.

Finally, you would need to find someone crazy enough to manufacture and deploy this biological agent, yet also competent enough to pull it off, which we have seen is no easy feat.

I am not sure if I would go so far as to say that the above is physically impossible. But without a very good argument, we should regard it as highly implausible that all of the above will come to pass by the end of the century.