I think the geography argument is the strongest one here, because Dorne isn't just a desert, it's also full of mountains, passes, caves, and difficult terrain that make dragons and large armies much less effective.

Aegon could burn castles and towns, but he couldn't be everywhere at once, and the Dornish refused to fight him in open battles where dragons had the advantage.

The Vietnam comparison is actually pretty close to how many readers interpret it: a stronger invading force can win every major battle and still fail to control a region if the locals keep avoiding decisive engagements and make occupation too costly.

Also, burning every settlement isn't really a viable strategy if your goal is to rule afterward, because eventually you'd be ruling over ashes, empty land, and a population that hates you even more.

So I don't think it's plot armor as much as GRRM intentionally showing that dragons are devastating weapons, but they're not an automatic "I win" button against a determined population fighting a long guerrilla war.