It’s just a rule of thumb that viruses evolve to become less deadly and more virulent over time. Viruses that manage to become endemic generally do become less deadly and more virulent when you look back in time because the historical observer is selecting for viruses that didn’t kill their hosts and go extinct.
But here is the thing - Mutation is stochastic. There is nothing stopping an endemic virus today from suddenly developing a lethal phenotype. Influenza has been endemic in our species since before modern humans existed, then the Spanish influenza shows up in 1918 and kills 20 million of us.
We can look back and say well that virus was an evolutionary dead end because it killed it’s hosts, but there are 20 million reasons why we should not rely upon that being a rule for predicting the lethality of any future viral pathogen.
There is recent literature in Marek’s disease showing that the mechanism of action of the vaccine to reduce symptoms reduces transmission in both the vaccinated and unvaccinated birds. Flock immunity if you will.
“Consequently, contact (unvaccinated) birds are less likely to develop disease symptoms or die, show less severe symptoms, and shed less infectious virus themselves, when infected by vaccinated birds. These results highlight that even partial vaccination with a leaky vaccine can have unforeseen positive consequences in controlling the spread and symptoms of disease.”
Mind you the Sars-CoV-2 vaccines are not especially leaky, or I should say no vaccines are 100% neutralizing but these are pretty good. If you go back to March 2020 when the first human test of the pfizer vaccine started the FDA was prepared to approve vaccine that was 50% effective … and we created multiple that are 95%+ effective. They are about as close to non-leaky as any vaccines have been.
Neutralizing protection is very strong but wanes over time as the immune response shifts to memory b-cells which remains a strong but delayed response. It just means that a reinfection results in a milder case as it takes a few days for the immune response to get back into top gear. And if we want to keep neutralizing response while viral transmission in the community is high, so we can try to avoid any disease, then a booster shot does that.