I've watched nearly every game of his career, and he is not worth 8m a year. He is not a legit 1c, and he's shown that his entire career. He is incredibly streaky, and has mental issues that affect his game.
You're correct that Colorado was crap for a few years while he was there, but part of them being crap, was having Duchene as a 1c.
I'm not thinking of power forwards, I am thinking about players like Duchene, who rely on speed and dexterity to perform. He will only get slower, and less agile.
He's broken 60 points once in his career, and he's had talented players to play with.
He's a decent all-around player, but isn't elite at anything. He was pretty fast in his first few seasons, but that is long gone.
I don't know a single person who's followed his career closely, that would want him at 8x8.
I'd be ok with 6.5×5, and even then, I'd have concerns for years 4-5.
I'm also saying this as a Duchene fan, who has 2 of his jerseys.
He's a great 2c at 6.5m on a team looking to contend, and who already has a legit 1c.