That's not a plan of campaign, that's a rant with a very dubious relationship to the reality of the Thedas setting.
Well when Bioware actually commits pen to paper and makes military logistics and manpower accountable to the realities of their own designed world you may even have a far point. But the amassing of strength, the building of Surplus and the construction of forces are evident to the lore, More or less you have a continent that has the forces to easily outnumber the Qunari whom according to Wot are consumed with this predesigned Eugenics programe to create the perfect species for their dogma.
Even with out the exacting totals if you by the Lore, you clearly see the Militaries of Thedas matching and exceeding the Qunari of the prior war in both number and the ability to mass forces, if the Qunari could match ability and number then they wouldn't have been broken, driven back and forced into that pseudo peace they agreed to after the first conflict.
Admittedly its all conjecture on my part, but it isn't as you descrbed an inaccurate rant, its just my conjecture on what Bioware has presented thus far, if Thedas and its...Northern Ally unite, they likely could exceed the manpower of the Add in that the longer the conflict draws, the more strain it presses upon the invaders to maintain their conquered lands and you have a scenario for absolute victory, after all they lost the Imperium originally because the Magisters combined mass slave revolts with military action, I don't see why you couldn't apply the tactic to the wider arena.
You force them to commit to battlefields with their reserves, you press supply lines and strongpoints and ultimately get them bogged down on multiple fronts while amassing the force to cripple their homeland, its a viable tactic that has seen much use in our own reality, not as much in Thedas given their wars for the most part have been one Nation against other, you have relatively few international wars, only a handful of events prompted agreements between the various powers that be, but this is one of them.