Mutations which improve transmission and longetivity will, in general, become more prevalent. Proliferation begets proliferation. You cannot transmit a strain if you are at home in bed from its severity.

Help me understand how a virus can replicate without being detected but can also increase its lethality.

We already have an asymptomatic and infectious incubation period. If that kept stretching longer then you are delaying the lytic cycle even longer? Or does propogation and ultimately viral load also happen as a slower burn? Eventually you will have damaged too many cells.