Yes, it absolutely is mortgaging the future.
Let's say, for argument's sake, that Gaudreau bounces between being a 65 and low 80's point player for his career as he has been so far. He could be better, or he could regress
Marner's upside is absolutely in that realm, if you doubt that you're in an extreme minority where the majority includes the publishers of these lists:
http://www.tsn.ca/tsn-hockey-s-top-50-nhl-affiliated-prospects-1.436669
http://bleacherreport.com/articles/2652458-ranking-the-top-50-nhl-prospects
So, for argument's sake lets say that Marner turns out and is more or less the equivalent of Gaudreau.
Let's also say that the leafs are bad next year again, and that throw in 1st round pick is Liljegren...and for fun, lets say he turns out as well....the most frequent comparable I've seen is Drew Doughty.
So, in this not-so-far-fetched scenario, we've traded:
Marner, who is as good as Gaudreau in this little exercise
The Swedish Drew Doughty
and just for fun, throw in a functional 1st line winger
looks like good asset management to me.
Here's an honest question: do you think the leafs did well in the Kessel acquisition trade?