Sure, and this is where the immune erosion comes into play: as vaccination rates go up, so does selection pressure for immune erosion. Re: delta, think of this

Sure, and this is where the immune erosion comes into play: as vaccination rates go up, so does selection pressure for immune erosion. Re: delta, think of this counterfactual — what would happen if a new variant appeared that is more lethal, more virulent, and that caused hospitalizations more quickly? You can bet the lockdowns would ratchet up. With Omicron, we didn’t even notice it until there was a random genome scan. Viral evolution rarely selects for host death — it’s just not useful as a goal. There’s also probably relatively weak selection pressure for symptoms, though probably casing sneezes in hosts helps with spread, it also acts as a signal that can cause the host to isolate and halt spread. (See: SARS v1.) From a selection pressure perspective, the ideal virus (among humans!) probably goes completely unnoticed for as long as possible. Humans won’t fight something they don’t even notice, and we probably have tons of these viruses everywhere and don’t even know about them.