It is bias because there's no strong indication whatsoever that someone else could be the big bad, and there are good reasons to believe he is, but many will conjure up unsubstantiated reasons to believe that Cory is not the big bad, ignoring, coming up weak counter reasons or simply straight up refusing to see what's in front of their noses, simply because they hope it's not true. Bias is like blind spots, you won't recognize that it's bias.
Cory is the big bad of DAI, the DA team planned this all along, likely since the very early stage of DA franchise. I find it difficult to believe they will change the big boss of DAI in the middle of its development, because DA is a franchise with a overarching story spanning multiple games, where certain things must happen - i.e. Corypheus bringing chaos to Thedas, get killed, but set in motion the next chain of events for their next games.
Aside from the moment where it was revealed Revan was indeed the main character in KOTOR, Bioware was never known to have very strong plot twist/revelation in their stories. They are relatively weak in this aspect, really - which is not as bad as it sounds because their story are usually good enough and delivered fairly well. Of course I fully expect one or two very strong plot twist/revelation throughout the DA franchise, but in general, they'll be few and far between.
The Cory situation is not handled well, from a 'surprise' or 'mystery' factor point of view. If they'd not shown that Corypheus had body hopped to Larius or Janeka at the end of DA:Legacy, making the player believe that Corypheus is truly dead, then reveal in DAI (when Cory is revealed to be the big bad) with a flashback that Corypheus actually body hopped before dying, maybe many would not be disappointed. At least that's my idea.
Some fans' theories like Dumat being the big bad and Cory working for him sounded better than Cory being the big bad. However, Bioware never truly excelled at such storytelling with multiple layers and depth. Dumat we know is dead, if he somehow turns out to be the big bad, it'll take a hell of a lot to explain how he isn't so. That will probably work for the regulars of DA, but not for new players to the franchise.
It comes down to how well Bioware deliver Cory as the villain in DAI. I personally think Cory can be a great villain.
BTW. The Mass Effect team is not the DA team, so whatever bad that happened in ME3 has not bearing on DAI.