If antibodies don’t last long, how does a virus “run its course”? The assumption on “run its course” is antibodies for the specific strain generated by the host are sustained, evidence is building that the resistance window is short (weeks vs months). So the strain could conceivably never “run it’s course” naturally as people could reacquire the same strain (I think this needs to be verified/documented, my understanding is the studies are looking at antibody levels and assuming as a result the virus could be reacquired vs documented cases proving it). If it does play out that way, we need to hope the vaccine(s) are highly effective.
Per the article below, flu antibodies can last a long time, it is the various strains that cause repeated sickness in the same season. Also the flu itself provides stronger antibodies than the vaccine, which might be the inverse for Covid.
https://well.blogs.nytimes.com/2016/10/28/does-the-flu-provide-better-immunity-than-a-flu-shot/