There are now multiple strains of SARS-CoV-2 (based on literature I’ve reviewed for my job in the last month) and it’s an RNA virus so it’s more prone to mutation.
I understand that nothing is certain with any particular virus, but my understanding is that they tend to evolve to less lethal versions - simply because if a virus kills people quickly and easily, it doesn't get passed on as much as one which doesn't.
Is this correct?
Let me start out by saying that I am not an epidemiologist or biologist, but I have had some direct involvement in programs to address epidemics. I think I know enough to articulate relevant points in layman's terms, but a scientist might say I know enough to be dangerously inaccurate. If the latter is the case, I welcome correction from technical experts.
Although your question about evolving to become less lethal intuitively seems reasonable, my understanding is that, in practice, it isn't consistent enough to be part of a practical solution. One reason is because the lethality of the virus is only one of many factors, and those other factors can counterbalance it. Others factors might include speed of transmission, ease of transmission, and whether there is transmission during an asymptomatic period. So, yes, a virus that kills its host is less likely to be passed on, but high lethality can continue to exist if other factors allow it to continue. Ebola is probably an example of a virus where high (and more importantly, rapid) lethality limits spread. Ebola has a very high death rate and kills quickly, but there isn't asymptomatic transmission and transmission pretty much requires direct contact with infected bodily fluids. Ebola hasn't wiped out populations in Africa where it has long existed; probably because it tends to burn it self out by killing off people before they can spread it widely. So yes, Ebola might spread wider if it mutated to become less lethal. But it could also spread wider if it mutated to act more slowly or more easily transmissible.
Another highly lethal virus is HIV, but it has spread far more than Ebola. Modern treatments are highly effective, but those treatments aside, it appears that pretty much everyone who gets infected with HIV will eventually develop AIDS and AIDS pretty much kills 100% (or at least it weakens your immune system to the degree that something else kills you). HIV/AIDS essentially always kills its host and it isn't easily transmissible, but it has spread globally (unlike Ebola). Why? HIV's "success" as a virus seems based on the fact that its very long period of asymptomatic transmission outweighs its lethality.
So yes, a virus becoming less lethal might make it more successful, but so could any number of other mutations. Mutation and evolution can take a lot of paths to "success," and sometimes (most times) they just lead to failure.