Tavares is not a complete non factor. Tavares' job in this series is to eat up Bergeron's time. His stats were always going to suffer in a series against Boston, but his presence has allowed Matthews to score 5 goals. Matthews doesn't look great either and was really asleep fro the first half of the series (I really wonder if he plays scared because of his shoulder), but Bergeron would destroy Matthews if he was the 1C and leave the Leafs with nothing after that. This Leafs teams vs this Bruins team (as in, not looking at last year's rosters) without Tavares is a complete disaster for the Leafs. That's why he's still a factor.
The problem is that you pay an $11 million guy to take over the series and not just take away the attention from other players. Bergeron will eat up most people, and he ate up Tavares his entire tenure as an Islander. And he would eat up Matthews in the playoffs all day. But that's the flaw in the Leafs' makeup. A lot of money tied up in players that aren't moving the needle enough. They are very talented, but are lacking that extra factor.