There is no selection pressure for Covid to become less lethal. None whatever.
Furthermore, no human virus has ever evolved to become less lethal.
Unfortunately this myth is often used by those who would like us to let Covid keep on killing, in order to protect their cashflows and for ideological and political reasons.
Isn't there though? Say Covid killed within seconds. Then it couldn't spread. The longer the host lives, the more other people it can infect. I would almost say that the opposite of what you wrote is true.
Rabies effectively has a 100% death rate and it can take years for it to kick in. There are cases of people suddenly showing symptoms 5 years after initial exposure, and once that happens, your brain basically rapidly rots and you drop dead.
We have no clue what the long term effects of any covid variants are. We can only speculate.
It's believed that each one of the other coronaviruses which are now endemic "common colds" began as a more lethal pandemic, the last probably being in 1890.
https://www.theguardian.com/world/2020/may/31/did-a-coronavi...
Yes, Covid is primarily dangerous when it manages to get deep in the lungs and cause pneumonia. It takes a week or two of being sick before it gets there. That's why you heard early in the pandemic how people were fine, until about day 7 or day 10 and then suddenly went to the hospital. Other coronaviruses can do the same thing. If you ever get a cold for 10 days, your immune system is probably pretty bad and you very well might get pneumonia. But you don't usually get pneumonia from the cold coronaviruses, because your body knows how to fight them off. When the 1890 coronavirus hit, no one had immunity and it was pretty deadly. But we have good immunity to that virus now (so we call it a common cold). As long as you can ramp up an immune response and kill it off before it gets a foothold deep in your lungs.
[1] https://www.asbmb.org/asbmb-today/science/051620/what-is-the...
Wikipedia theories are as reliable as usual. (Not reliable)
But that may or may not be what caused the black death in medieval Europe, in my view. There's reason to think it might have been a really nasty hemorhagic virus like Ebola. See: https://history.howstuffworks.com/historical-events/black-de...
There had been several pandemics of black plague, but at some point it didn't come back. Why?
Maybe there was a bacteriophague virus which caused Yersinia Pestis to be lethal? (Hypothesis) But why would it be gone?