I can remember when DA2 was released in 2011 (I think?). Many people raged and were very angry at Bioware. They wanted exploration, open world, customization of weapons and armour, crafting etc. And when Bioware gives these people all they ever wanted, people still complain? Bioware this times around gives us - it seems - a final antagonist to be defeated and the game then ends when we have defated this antogonist. Something most people have wanted - it seems - since Bioware relased DA2 way back in 2011. And Bioware still gets complaints from people? I don't get it. But this just shows, as my mom, always said: you shouldn't give all the people what they want all the time. They'll just complain...
On topic:
I'm fine with the big bad in this game being who it is. (if it is?) I have sort of guessed it mysefl somewhat....since the ending of the Legacy DLC. To me, it was clear that we would see the person we're talking about here again...
Edit:
As far as Dumat goes, yes, this archdemon maybe dead. But who is to say that the Elder One isn't another old god?