Leafs looking to sign Tanev to 6-7 years? Lol, no way. His last contract which was signed when he was 4 years younger was fair (4 years $4.5m). 6-7 years for this 34 year old band aid would be seriously hilarious.
The way I look at it is the Leafs have effectively peaked and backed themselves into a corner trading most of their picks and good young players, so this is an effort to extend the peak and by offering more years than anyone else you try to get the AAV down so you jam pack the roster as best you can for the next couple years. It's a rob Peter to pay Paul kind of move that hamstrings the next GM when the roster invariably starts declining.
What I will say is unique about Tanev is he has gained a reputation for being one of the best mentors a young D could ask for, so even when he's overpaid in years 4 through 6-7 he's still an asset in helping training up the next batch of Leafs D.
This isn't me saying it's a smart move, but it's an understandable move when you are coming from a place of desperation to get more out of this core group.