> we aren't immune against flu, is there any reason to think we could develop (long-term) immunity to COVID-19?

Spanish flu became less lethal in subsequent generations[1]. Helps with the survivability of the virus if it doesn't kill the host. So, the strains with less lethality win out.

We don't need to be immune for the situation to resolve. Simply co-evolving with the virus is sufficient

[1] https://demystifyingscience.com/blog/2020/3/4/patriarch-of-p...