Thats doubtful IMO, if the Archdemon were just really powerful Dragons, they wouldn't be capable of leading a Blight, High Dragons like the one located at Haven just did what you expected of a monster, it didn't cause an organised invasion of nations of Thedas, it just unleashed a massive rampage every few centuries and went dormant again. The Archdemon however is capable of communicating to the Darkspawn and use them effectively (He was smart enough to draw your entire party to Redcliffe with a few Darkspawn while the main horde ambushed Denerim). Demons in the Fade do not possess such cunning, many of them do not understand the mortal realm at all and believe that their own power would suffice, hence they never use any tactical maneuvers, they just attack and attempt to change any mortal into an abomination. Uldred is an example of this, as does the Desire Demon inhabiting Connor. You also recall how the Old God is "corrupted" by the Darkspawn taint, that doesn't just happen to demons, there is just too many seperating them apart, Demons call out to mortals in the Fade, the Archdemon does no such thing, and calls out to the Darkspawn instead. They are completely different beings, if the Archdemon happened to be just another form of Demon, then there should be demons within the Fade capable of understanding the taint in the Grey Wardens, which they don't. The Tevinter Imperium were hardly "primitive" at that point either, you don't have a grand empire, with powerful magisters and end up being primitive, they worshipped the Old Gods like you said however because they had immense power, and it's obvious their power would've far exceeded that of a normal Dragon. It was also said that the Old Gods taught them magic, whether its true or not, most myths originate from the truth.