Mutations will optimize for transmission, but once there's a high level of immunity against this strain (all the variants are still one strain) then the virus will face the pressure to mutate to achieve actual immune escape. There's 20 epitopes on the spike that need to change, and some of those should come at a cost to fitness, and eventually it should face a choice between trying to increase its transmissibility to find immunologically naive humans (where delta variant is probably reaching its limit) and to better transmit in recovered/vaccinated individuals due to immune escape.