They're both AAA games made for the masses. The Witcher 1 and 2 has sold over 6 million copies, which isn't far behind Dragon Age 1 and 2. Given the massive budget, The Witcher 3 is probably aiming for 5+ million alone. The audience of The Witcher might be somewhat different to that of Dragon Age, but a lot of them (such as me) will be playing both, but only one at a time.
Oh, I'm not saying they won't sell well - I'm saying they're not aimed at the masses in terms of design and the nature of the storytelling.
If it had been the original Dragon Age with the old Bioware developers, it might have been a different story.
But Dragon Age 2 was clearly a game designed for mass appeal and the mainstream audience. They just messed it up big-time, but that's another story.
That said, one is a single character game with primarily action combat and the other a party based game with "tactical" combat.
It is beneficial for both BioWare and CDP to not release at the same time. They'd rather have "AAA RPG monopoly" for 3+ months than forcing people to choose one over the other. It's good business to target a period noone else is targeting, as they'd attract practically every RPG fan in the world then.
Sure, if you believe people stop buying games they want to play because there's more than one available.
I consider that an out-of-date perception that hearkens back to the old days where retail was the only option, whether you were a core gamer or not.
For DA:I - which is clearly a mainstream RPG, it might be a problem because people who're not dedicated RPG fans might not get a cheap key or whatever, and still think it's smart to buy retail versions.
But The Witcher 3 being seriously threatened by DA:I? I just don't see it.