Through the power of Plot Armor, all things all possible.
Well, here's the problem. With dragons, you can trash large cities and castles on a large scale. It's not
completely risk-free (as Queen Rhaenys found out rather fatally in the First Dornish War), but you can do it.
The thing is, in any medieval society, most of the population doesn't actually live in those cities. They live in the surrounding villages that grow food (in a desert this may only be happening around the oases and rivers but it still happens), which is then shipped into the cities to sustain them. Often the cities are actually not self-sustaining in population, with disease outbreaks regularly killing large fractions of the city. Then the place has to be replenished by migrants from rural areas where there are nearly always
some families an able-bodied family member that they can't feed in a bad harvest year for lack of land, and they might as well tell the boy to go into the city to seek his fortune and hope for the best...
So even if you burn down every city in Dorne, you've probably only actually killed 5-10% of the population, especially since people will be fleeing those cities back into the countryside as word spreads of what you are doing.
If you want to go farther and burn every
village in Dorne, you're going to be at it for years and years of nonstop hunting down and destroying villages. Because there are
thousands of the little things, given that the overall population of Dorne is at least a couple of million. And some of them are very remote, and you're working your dragon pretty hard just flying all over the place looking for them, and you (and the dragon) have to sleep some time and that's a bad time to be ambushed by Dornish warriors, and so on and so on.
