Thornton didn't play in the playoffs this past season due to injury and the year before only played 4 of their 6 games due to injury. In those games that he did play, he was playing on a torn MCL and ACL in the same knee. Tavares would be a massive upgrade to what they have actually gotten out of Thornton down the stretch and in the playoffs for the last 2 years. Tavares might be a lateral move from 2015/16 Thornton, but the rest of the team is arguably better than that group. That team made it to the Final and has since added E Kane, Lebanc (40 points at 22 years old), Meir (36 points as a 22 year old) and seen noticeable improvement from Demelo and Dillon. They have lost Boedker and Marleua, but I think the improvements outweigh the losses due to the increase in speed and youth. Plus, even assuming Thornton is just a 40 pt player at this point, he would still be an 'upgrade' if Tavares is just seen as replacing Thornton.
No team is a lock for the Cup, but I don't see any argument that adding Tavares isn't a big upgrade.