That was development with more complexity. This just sounds naive and silly. But for one guy's death, everything between Orlais and the Dales would be kumbayah?
Drakon and Ameridan were friends from before there even was an Orlais.
Ameridan intended to bring the Dales to support Orlais against the Darkspawn.
If he had done so, it's possible relations between the two nations would have been more amicable. Maybe he would have introduced a synthesis of Andrastianism and the elven religion which would have appeased the Chantry. Maybe he would have lifted the elven isolationism and established mercantile connections with Orlais making an invasion less likely or an occupation more merciful.
Would it have guaranteed peace everlasting? Probably not. Neither Drakon nor Ameridan were immortal and the differences between the two nations might have made war unavoidable.
Regardless, there is nothing naive to wonder about what could have happened had a different leader been at the helm of the Dales. The DLC never tries make the argument that all that was needed for "kumbayah" was Ameridan. Only that things might have been happened differently.
In fact, the very memory where Varric accuses the Hakkon of being indirectly responsible for the destruction of the Dales has Ameridan admit that a number of elves wanted to abandon Orlais to the Darkspawn which is what happened.