I was under the impression viruses became less lethal because if they became more, they'd kill off their hosts and be less likely to spread.
There's a chance you get some super virus that becomes both more transmissible and lethal, but that is very rare (from what I can see).
Only if the lethality is negatively impacting their fitness. If it can spread to another host quickly, who cares if the previous host dies. There's no selective pressure against it.
Which is why Delta is scary. Not just because of the higher viral load, but new strains could be more lethal and still not be selected against.
And let’s not forget about Mareks disease, where a non-sterilizing vaccine is thought to have contributed to a massive increase in virulence, killing nearly 100% of unvaccinated chickens now vs causing mild disease decades ago.
With a different presentation of disease where symptoms and spread occur concurrently then there would be a pressure on the virus selecting for strains that arent so lethal a carrier has no chance to spread. In covid’s case, that pressure doesn’t exist as far as i can see?