The need to achieve immune escape can also pressure the virus to evolve antigenically which can promote transmissibility through escape, while impairing ACE2 binding or endocytosis compared to currently circulating variants.

So if the ancestral strain + variants all cannot spread epidemically any more due to the build up of immunity in the population, then a strain which achieves immune escape and an R0 of > 1.0 would be favored even though the virus might be much less "fit" and less virulent in a totally naive population.

Clearly though the major adaptation is going to be on the human side with most of the population eventually acquiring T-cells, which will greatly decrease the virulence going forwards.