I agree as well. I liked Kane as a player, but he was going to be a UFA and either he was going to leave and we would have gotten no return, or he would have commanded a huge salary to stay. Whether or not his personality was a good fit for what the team is trying to do in the future is debatable, and he has had some injury issues, but the UFA situation is undeniable. Theoretically, we still could re-sign him after the playoffs, when he will be a UFA, but most likely, he will get more money elsewhere and/or elect to play in a more cosmopolitan market than Buffalo. I love Buffalo and a lot of NHL (and NFL) players do, but it isn't for everybody.
The Kane situation is quite similar to the situation the Bills faced with Sammy Watkins last season. In the end, the Bills made the right move by trading him. They got a decent return (2nd round pick and the services of a decent CB for 1 season) for a player they were certain to lose in Free Agency for nothing. He ended up getting something like $16 million from the Chiefs, and there's no way the Bills would have been able to match that, given their variety of needs and their salary cap situation. And even if they wanted to match the Chiefs' offer, there's no guarantee that Sammy would have accepted it.