Sure we don't know.
We also don't know that what Duchene will bring, at a minimum is going to be enough to get us the cup.
There are no guarantees. I'm all about extending the window for as long as possible. Giving multiple chances at the cup. The best way to do that is to have elite talent at key positions. Our LHD is absolute shit in the minors, and Markov isn't getting younger. Emelin-Beaulieu-Barberio is not a good depth chart at the position, but an elite talent makes it look a lot better, and extends the window for a team that has Galchenyuk at just 22, Price relatively young for a goalie, Max isn't old, Gallagher, Danault, etc.... I don't think our window is closing so fast that Sergachev who is 1-2 years away IMO can't be a part of it.
And to me, its VERY, VERY, VERY hard to get elite talent. Its hard but not impossible to get very good, non-elite talent. I would never give up potential Elite talent for very good but less than elite.
I understand that philosophy but I dont subscribe to it. We've been making the playoffs and having moderate success for years. It's not enough for most. I'm a Broncos fan. They won the SB last year and when they missed the playoffs this year I got over it very quickly. A championship can satisfy a fan for years.
I don't think its crazy at all to hold on to a young prospect who could be elite instead of moving him for a very good but not elite player.
That's the type of trade that gets teams in trouble all the time. Its the kind of thinking that led to a number of Leaf trades over the years, the type that kept them a laughing stock, and its the kind of thinking that gets you into a McDonagh for Gomez deal. Lets be clear, I don't expect Duchene to turn into Gomez. But you are doing the same thing, moving a potential number 1 defenceman, for a 60 point centre.