Same as with what probably happened with HCoV-OC43, another betacoronavirus very similar to SARS-CoV-2. Evidence indicates that it caused a worldwide pandemic that killed a lot of people starting in 1889. Now it's endemic. Most of us catch it when we're young and healthy which gives us partial immunity later.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7252012/
https://sfamjournals.onlinelibrary.wiley.com/doi/10.1111/175...
Eventually almost all of us will be infected by some variant of SARS-CoV-2. Fortunately the vaccines are very effective at preventing death.