I remember reading they expect immunity to last as little as 6 months for Covid-19, found this paper about the SARS outbreak of 2002 which was also a type of corona virus, saying patients had an average immunity of 2 years.
As far as I'm aware, antibodies don't stick around after they're no longer needed... They're produced by B cells when an infection is detected, and after an initial infection, some B cells that learned how to make effective antibodies stick around in your body as "memory B cells" - these will produce the same antibodies again upon reinfection.
Counting antibodies a few years after a SARS infection doesn't seem like it would tell you much about immunity.
SARS-CoV-1 completely disappeared after two flu seasons and hasn't been seen in decades now.