Your understanding of the immune system is not quite correct. Just because you can't detect antibodies doesn't mean you lack immunity.

B-cells can stop producing antibodies and reactivate production in response to an antigen. T-cell memory appears to be quite robust for coronaviruses (and this one in particular)

To establish a lack of durable antibody response, challenge trials would need to be done, where the host is challenged by the virus (or something that looks like the virus like it's spike protein). These trials have not been run.

We can only guess without data specific to SARS-COV-2, but for seasonal coronaviruses, antibodies disappear after ~6 months and reinfection with the same strain can occur after 1 year.

It isn't entirely irrelevant. If you can get reinfected but not get sick twice, it's as if we reached herd immunity at 100%.

And it's perfectly normal that antibodies don't stay in the bloodstream forever. The immune system remembers how to make them and can produce new ones when needed.