I would be very surprised if the 2019 first is as good as Duchene.
Duchene is 27, and Ottawa acquired him with under 2 years left on his $6 mil AAV deal before he becomes a UFA. There's a decent chance they get just 1.8 seasons of Duchene, and even if they do resign him, it'll be an always-risky UFA deal - probably for tonnes of money and term, in his 30s for almost all of the deal, these contracts are always major risks and often work out poorly.
The 2019 1st could easily be a top 5 pick, and top 5 picks turn out as good or better than Duchene decently often. But even if he's significantly worse, he'll be an 18 year on a dirt cheap rookie deal for a few years, then a still-reasonable RFA deal for many years after that. He doesn't have to be as good as Duchene to be equally (or much more) valuable, because you get him for ages at a good-to-great price, while with Duchene it's either just 1.8 years, or longer, but on a very risky, very expensive UFA deal.
I think Ottawa made this deal thinking they'd be a playoff team, and the pick would be a mid/late 1st. Given Ottawa's very poor performance this year, and the likelihood of the pick being very high, I think they'd at this point deal Duchene straight-up for the pick back, while Colorado wouldn't.