That's naive, at best. There is a civil war because a Bann who wasn't present at Ostagar questioned the timing of Loghain's retreat.
That's a bit of a stretch. It's a long way from saying "we won't do what you want simply because you tell us to" to actually going to war. More importantly, there's no way Teagan could even have fought that war, because he was holed up in Redcliffe immediately afterward with no outside communications.
An entirely valid alternative reading is that Teagan represented a suspicious opinion among many banns, one that was eminently justifiable. They had seen their fellow nobles, their King, Loghain, and a bunch of Grey Wardens march into the Wilds to fight darkspawn; they then saw what was left of the army running hell bent for leather
out of the Wilds, minus a king, all his Wardens, and a sizable chunk of the army. Loghain blamed everybody but himself for the defeat and immediately set about making himself
de facto ruler of Ferelden.
Under those circumstances, it would be entirely reasonable to question Loghain's account of events. So too would it be reasonable to point out that he wasn't the leader of the country and the bannorn were under no obligation to simply follow his orders if they weren't inclined to do so. And it would be reasonable to question, if Loghain failed to defeat the darkspawn with the forces he lost at Ostagar, why he should be entrusted with anything else. It's well and good to point out that Loghain (probably) had no direct knowledge of why the Wardens were necessary to deal with the Blight (a common tack of Loghain's biggest fans, including, if I'm not mistaken, you yourself), but it's a little tendentious to then turn around and say that the bannorn should simply have done what he demanded without questioning, as though Loghain were any different in this respect from the Wardens.
Teagan's comments were not intrinsically an act of rebellion. Perhaps, given the rapidity with which Loghain seized power, they were instead a suggestion to negotiate: they would not bow to Loghain
simply because he demanded it, but perhaps they would do so
if he gave them some sort of positive reason. A charitable reading of Loghain's actions after Ostagar is that he wanted to restore order quickly and unite the country against the (darkspawn/Orlesian/nug/whatever) threat, but even if this was the case, the actions he took to do so were functionally indistinguishable from a
coup d'état, and people generally need more to go along with such a coup than "because I said so". And, even if Loghain were a genuinely disinterested actor, surely even his most die-hard supporters would concede that he was a singularly unpersuasive politician; the civil war may simply have started because he refused to explain his actions to anybody.
Anyway, it's extremely easy to explain the events after Ostagar without saying that that the bannorn launched a suicidal war against Loghain because reasons. That doesn't mean that you're wrong; perhaps the bannorn did exactly that, but there's simply no evidentiary basis for it one way or the other.
Not that an evidentiary basis would mean much anyway, considering the extent to which the developers alter and retcon the story based on their whims at the time. Ostagar has gone from being Loghain's fault (
Origins) to Cailan's fault (
Return to Ostagar) to Loghain's fault again (DA2) to an easy joke at the fans' expense (Threnn in
Inquisition) to an event with an unknowable explanation that the devs are clearly interested in keeping ambiguous (Solas in
Inquisition). They might very well decide something else tomorrow, in service of a different story. Trying to determine objectively correct lore explanations for virtually anything is a fool's errand.