I don't know why the inaction on the part of the nobles in protecting their people is a validation for Teagan using Sophie to criticise the Inquisition. As I understand the history, Sophie lost the throne to her cousin Arland and joined the Grey Wardens. Then Banns, who presumably must have voted Arland in at a Landsmeet, regretted their decision because he proved to be a tyrant. Instead of having the guts to simply remove him themselves, they approached Sophie and asked her to intercede, persuading her to head up their coup. She gets killed and the Grey Wardens get banished from Ferelden. Seems to me that the people most responsible here were the Banns, people like Teagan, who voted in a tyrant and then wanted him gone, presumably because they were suffering under his rule.
Whilst World of Thedas suggests that Fereldens are largely expected to take care of themselves, with regard to law enforcement, it also says that nobles must earn the respect of the common people. In another source book it states that "The primary purpose of the nobility of Ferelden is the fight for their people against all threats - human, darkspawn or otherwise." It is basically their job. Outside of their Keeps they do not own land, the freeholders do. They give their patronage to nobility in the form of tithes and if they feel their bann or arl is not coming up to scratch with regard to their protection, they can change allegiance. This is done on an annual basis.
This is why it seemed strange to me that the nobility weren't making more of an effort to protect their freeholders, most of whom had actually been displaced by the events at the beginning of the game. One war table mission actually had a noble complaining about the refuges on his land and asking us to remove them. Cullen, the Ferelden native, suggested we help the refugees instead. So you see from that point of view the Inquisition were perfectly entitled to hold Caer Bronach, since it had been captured by bandits with no local noble attempting to reclaim it, the citizens of Crestwood would have been overrun by undead if we hadn't helped and the bandits were severely impacting on trade. The correct response to Teagan under Ferelden traditions would be to suggest that the freeholders in Crestwood decide who they wanted holding Caer Bronach. Since we had been doing the duty that should have fallen to Teagan, or whoever else was responsible for the Keep, the freeholders would likely have opted for keeping the Inquisition.
Of course, by taking the Aggressive Disband option I was allowed to point out this fact to Teagan "It wasn't a carefully worded agreement that saved the people of Ferelden". That gave me a degree of satisfaction as I walked out on them all (I like to think I was actually giving them the two fingered salute as I waved my hand in the hair) but I do feel sorry for the people of Ferelden. They were the ones who suffered at the hands of the darkspawn, not the lilly livered nobles (remember how it was left to the Chantry and Templars to help the people of Lothering after they were abandoned by their local lord). They were the ones trying to rebuild and whose efforts were being destroyed by the events in DAI. I dare say that next time the s**t hits the fan, no one will be shifting themselves to help the freeholders either. No point in looking to my ex-Inquisitor either; he's got bigger fish to fry ("Now if you'll excuse me I have to save the world, again.)