Note that no extinction is absolute, so, even here, souls are preserved but they incarnate into wild (neutral) bodies. Co-evolution of a polarized soul with a neutral body will result in a
less polarized hybrid, thus subduing cancerous tendencies. If these are further energetically constrained, they will be benign.
With cycles of reincarnation these souls would then be integrating into growing neutral human society as neutral individuals.
On the other hand, if population does not separate, due to polarized individuals being a majority, this will certainly lead to destruction of all life and death of the planet - unless
nature (immune system of the planet) does have some other cure to cancer and will react to specifically target cancerous individuals (ie. through viral mechanics and/or introduction of
intra-terrestrial lifeforms that prey on humans).
Most likely, such mechanisms do exist in a planet (COVID-19 is then likely a precursor to more specialized viruses), it is only a question in what measure they will be exercised and how precise
will the targeting be - after all, difference between a good and bad (cancerous) human is, like in case of TGF-β proteins in humans, more mental than physical.
In case of viral mechanics, DNA could have a role in this [natural] selection, while, in case of predators, fear is likely a strong factor. In any case, synchronicity should have a strong role.
Therefore, separation is probably a less painful solution.