That's true - but that shouldn't remove an obligation to be vaccinated, because immunity, whether natural or through vaccination doesn't last forever.
As I understand it, most peoples' immune system has some natural (innate) antibodies and NK cells. Higher quantities when young. These can prevent symptomatic infection, and are as effective if not more so than vaccine-induced adaptive antibodies and T cells. What I think may be missing from this debate is the ability of a person to utilize his innate immunity. Assuming the memory B-cells from which the innate antibodies arise (google B1 memory cells) hang out in the lymph nodes, there will be problems if someone has poor lymph drainage, for example from the head and neck, or the chest, because that person won't be able to utilize his full quota of innate immunity.
So just as one person might rely on repeated booster shots to give lasting immunity, another person, who might do yoga stretches daily or have regular massages, might be repeatedly exposed to the virus but his innate immune system is healthy enough to deal with it each time.
Also, if a person becomes symptomatically sick and recovers, they will usually develop memory cells which, on re-exposure years down the line, can then produce large quantities of specific antibodies in a short time, thus reducing disease severity.
So there is no need to force everyone to be vaccinated.
And there are other factors that influence whether or not exposure leads to infection. Poor dental hygiene for example has been linked with increased severity of Covid illness. Then there are things such as poor diet, and chronic stress, which lead to suppression of the immune system.
