I think it's a key point to mention that it's more that the hosts will evolve, i.e. develop immunity or die off, rather than the virus. The virus will continue to mutate but mutation activity is invariant to lethality (up to a certain point where lethality precludes transmission - where SARS-Cov-2 is far away from). In fact, because there is a correlation between viral load and lethality, the virus will actually get less benign on average in the short term.