Prior to COVID I seem to remember that the point of a vaccine was so that you didn't get the disease at all. That's where the herd immunity argument comes from. If 85-90% of the herd has immunity (immunity meaning you can't catch it at all) then even the people who can't get the vaccine are protected because the susceptible are too spread out for the infection to spread.

Do you see how this absolutely requires not just a lowered infection rate with lowered severity of infection? You need a less than ~10% chance of infection in the vaccinated/otherwise immune for the math to work. If the chances of infection after vaccination are still 80% then there is still a large reservoir of potential carriers and you don't have herd immunity.

For most of history what people expected out of a vaccine was immunity from infection not still getting infected but with less severity.