Do you have a source for this? I don't know much about immunology, but I know a little bit, and I don't think it's that thoughtful of a process. We have antibodies for all sorts of diseases, certainly not just for things like smallpox. This article suggests colds are endemic because they mutate too quickly for antibodies to be effective: https://www.technologynetworks.com/immunology/news/why-dont-... but we still make antibodies for them.
We make antibodies, that's not the analogy I'm using, and I am studied in immunology. Some diseases are so harmful, our body keeps antibodies floating around perpetually in case we encounter it (e.g. smallpox). Coronaviruses are mild enough that we can "remember" it with t-cells and spin up antibodies as needed. Part of what bones do is this antibody production process, and it requires more energy than vital organs.
