The Dales were perfectly happy sitting back and watching the world burn. It is only fair that the world sat back and watched as the Dales burned to the ground.
Ah, resentment, what a powerful motivator.
I don't agree with you that the Dales "deserved" to be destroyed just because people generations before them didn't help the Orlesians during the Blight. Yet it can't be denied that in-universe is something that is taken into consideration.
It's said that the Dales refusing to help led to bitter relationships with their neighbours that ended up in war. Even if it wasn't the main cause, it helped to fuel the flames of war. Tevinter and Orlesian blatant treason after the Third Blight sparked Marcher nationalism. It's also likely that Tevinter didn't help the rest of Thedas during the Fourth Blight because they had just suffered several Exalted Marches against them, so they weren't in a good mood. And modern Nevarra was born after Marchers and Wardens managed to defeat the Archdemon by themselves, without aid from the empires. Also, we know what happened to Loghain in DA:O.
But I insist, even if in-universe is an understandable feeling, it shouldn't be condoned, because people can learn. Tevinter defeated the First Blight, Orlais the Second, Tevinter and Orlais the Third, Marchers and Antivans the Fourth, Ferelden the Fifth. And the Dalish too learned the lesson and promised to help.
Truth be told, only the Orzammar Assembly didn't learn much from their mistakes. Well, maybe Bhelen was right after all.