Knies at 7x7 coming right out of his 2nd yr of the ELC would've been an overpay, but lots of teams have made bets like this to good effect. Since Shanahan, when they've had worthy options, the Leafs have no been able to do so. Once he started his third year (alongside stat generators like Matthews/Marner), there was no reason for him to even look at an offer. The key was to blow him out of the water with an offer of stability before he hit the ice.
Tavares, yeah, I get the age factor, but age usually cuts into footspeed and Tavares never had footspeed to begin with. Get term + deferrals to bring the cap hit down before something stupid like Brock Nelson's contract happens, especially if the Marner trade nets a proper 2C already, taking Tavares' primary leverage off the table.