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.

No, there is no "evolutionary pressure for all diseases", only for those where lethality precludes transmission, i.e. there is some cap at a some max.

Which is not the case for SARS-Cov-2 - viral titers peak after 2-4 days and population-weighted IFR is relatively low.

Isn't that what happened with the 1918 pandemic?

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.

We cant stop it from spreading during its asymptomatic phase.

Uuuhm... no?

Evolving implies some change to the genetic material, compared w/ prior generations.

Viruses can do that, and quite quickly, we can't.

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?