if you want a **** ton of salary retained you need to add more value

That's not even close for a half retained Duchene, The value is far off.

Avs don't have salary cap concerns the next two years. The retained salary is irrelevant to them. Back in the day, teams sometimes sent straight up cash in a trade. Think of this as the Avs buying assets. Bottom line is getting two top prospects, who could both contribute right away, and a high draft pick. Mission accomplished.