I think that is narrowing the statistical sphere. We have no precedent for how Kane performs with better quality linemates. Kane has never played with a player like Thornton, nor has he ever had a Norris winning offensive defenseman on the ice with him at all times.
I dont think it's naive at all to think that a good player will achieve more with better quality linemates. We have seen that on this team, around the league, and I have seen and experienced that at every level of competitive hockey i have played. Look at Matt Moulson. Dude was a 30 goal scorer with Tavares, then he went to the abyss of Buffalo and fell off a cliff. that's one example of many.
Point being, there isnt a strong enough reason to be pessimistic about a guy who has played exactly 0 seconds of a contract. Hate the contract all you want (i think everyone wishes it could be lower on both sides of the / ), but at the very least be neutral going into the season.