The thing about COVID-19 long term immunity is that it's not just about how long the antibodies persist for but also how useful the antibodies are in preventing re-infection. Just because someone has fought off infection that doesn't mean they've produced neutralising antibodies.

If someone has produced only non-neutralising antibodies to the SARS-CoV-2 virus, being challenged with it again would result in them at least being carriers for a while - if not symptomatic again.

It has already been confirmed that the individual antibody response to this virus is highly variable amongst patients with well-functioning immune systems. Bottom line: 'herd immunity' is not going to halt the spread of the virus amongst the whole population.

Because of genetic sources of variation in human immune responses, some people with well-functioning immune systems will not produce a strong neutralising antibody response to the virus. (Vaccines on the other hand can be designed to encourage a specific type of response.)