Virus' differ in many ways but I am no biological or medical expert, hence me acknowledging your point but highlighting that they may not be so easily comparable. Natural herd immunity in my opinion works well for infectious but not deadly virus' like the flu virus on the one end of the scale while it is never even attained on the other end of the scale for virus' like Ebola. I think measles, mumps, smallpox and probably SARS-CoV2 sit somewhere in between these two scales where they are easily transmitted, questionably deadly but the bulk of people suffer from severe symptoms. Natural herd immunity, while it has been effective for millenia according to some, leaves a lot of unknowns with new virus' and collateral damage which vaccines manage far better.