Technically I-35 doesn’t run all the way to Canada, but there are other major roads from Duluth to the border.
Not really. The North Shore of Lake Superior is more or less a dead end. There isn't much of anything in Canada above it. The biggest road north from Duluth is Hwy 61 along the coast of the Lake. It's a narrow, winding two-laner. Nobody would pick it for heavy traffic if there was an alternative.
The core of Canada is the interlake region of Ontario. That's where most of the people and industry are. Few people will ship anything from Toronto to Guadalajara by going through Duluth.
State Highway 61 (mostly 2 lanes) is the primary route to the border from there. Traffic volumes are not terribly high there, and any I-35 extension would be more due to regional demand than NAFTA traffic.