In "The Stolen Earth" and "Journeys End", we discover Davros and the Daleks heinous plan, not just to destroy a planet, solar system, or galaxy, but to to destroy all of creation... the entire Universe, and every single alternative universe as well, so that, in the end, there would just be Davros and the Daleks left. It wasn't clear but possibly also Earth and those other planets would still have survived as well.
What happens next? Davros and the Daleks destroy all of Creation. Then what do they do? There's nothing left to conquer, nothing left to destroy. It's just them.
Does Davros take up poetry and painting? Does he line up his Daleks in different patterns and take pictures?
Does Dalek Caan write a novel detailing his time in New York for other Daleks to read?
Assuming the planets were still around, maybe Daleks would have fun chasing and shooting humans on Earth, but in the end there would probably be none left.
I guess you can ask the same question of lots of this type of story: Evil dictator threatens to take over or destroy your way of life. EG, The Borg, if they assimilated Earth and the Federation, then what do they do? With TSE/JE it's simply taking it to the nth degree.