No one plays TES games for its story, or horrible voice acting. People love it because its a sandbox, and has mod support. Nothing more.
DAI was as perfect of an RPG as we've ever had this generation, or the last. The combat is incredible, it took the best parts of DA2 and DAO and combined it into a superior combat system.
The roleplay in TES games is ALL in your head. The dialogue options are 100% linear. DAI has the most roleplay options in any Bioware game to date, and it's all about how you express your characters views of the world, people, and factions around them in addition to your class and racial background. Like I said, Skyrim just can't compare.
So sales only matters when it suits your argument, that isn't a persuasive argument.
Played it for the story. I wanted to know what was going on in Tamriel. Did you check how many bought the game on Xbox and PS3? Those customers can't mod their games, you know. So your mod argument falls utterly flat.
Combat is not incredible. You are limited to 8 abilites, and you can barely see a thing. It is more like Diablo 3 than DA:O and DA2. Including spawning enemies. Nothing incredible about something that has been that dumbed down. You can't even set up tactics in the game.
And poor voice acting? Max von Sydow, Christopher Plummer, and plenty other solid actors = poor. Wow.
If an rpg sells poorly, when it has had a ton of marketing to back it up, it is because it is a poor game. Simple as that. DA:I isn't a poor game, and neither was Skyrim.