Very difficult to get herd immunity up to a decent enough level without putting the vulnerable portion of the population in great danger though. Most countries are approaching something like 0.1% or 0.2% of the population having been infected. Herd immunity requires 70% - 80% immunity. About 350x - 800x more than what's happened already.
Also the whole thing presumes that immunity is lasting. Some of the data is showing that may not be the case.
The level of herd immunity needed is related to the R0 which is a function of behaviour. This is a really complex topic to discuss, but it is only one we can have once an attenuated strain is found.
Of course the other factor is who is a vulnerable person is a factor of the pathogenicity of the viral strain.
Also the whole thing presumes that immunity is lasting. Some of the data is showing that may not be the case.