It's not that the virus will necessarily evolve to become benign, but rather that most people will get infected when they're young and build up natural immunity which protects them later in life. This is what already happens with the other 4 endemic common cold coronaviruses. Each of them probably killed a lot of people when they first emerged.
This is the correct answer and should be highlighted more. The virus doesn't evolve to become benign, it's the hosts that will evolve. In addition to the endemic coronaviruses that you stated, another example are endemic/seasonal influenza strains vs. new strains.
(Yes the virus will evolve but in general mutations will optimise for transmission and be indifferent to lethality.)
Mutations will optimize for transmission, but once there's a high level of immunity against this strain (all the variants are still one strain) then the virus will face the pressure to mutate to achieve actual immune escape. There's 20 epitopes on the spike that need to change, and some of those should come at a cost to fitness, and eventually it should face a choice between trying to increase its transmissibility to find immunologically naive humans (where delta variant is probably reaching its limit) and to better transmit in recovered/vaccinated individuals due to immune escape.
This is not exactly true. Keeping your infectious host alive longer, will increase transmission.
This is why there is an evolutionary pressure for all diseases to keep your host alive, and why some diseases/parasites ultimately evolve to be less harmful/lethal.
> This is why there is an evolutionary pressure for all diseases to keep your host alive
Lethality should be distinguished from "keeping the host alive." For example, HIV is more than 90% lethal (untreated) but can take a decade before killing the host.
It seems like lethality can become a first order evolutionary driver though by turning our focus on it and quickly smothering any "hot" virus but allowing "warm" virus to spread. Can't humans play a part in the evolutionary process of the virus?
In theory yes - and that's what's happening in many countries. E.g. in Germany there is a differentiation between high risk areas and virus variant areas, with different testing and quarantine protocols etc.
However on a global scale only some countries are sequencing some of the time, and only in humans (with ad-hoc exceptions)! Which is simply not enough.
Yes. Hopefully, vaccines confer the same immunity as exposure during youth. Based on their efficacy, it seems they’re quite close, at least for current strains—even with delta, the overwhelming majority of breakthrough cases are benign or totally asymptomatic, just like with a common cold.
so what happens if kids in lockdown now (e.g. australia) grow up w/o exposure to the common cold? is it suddenly going to become a big deal for them later in life?
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7252012/