I'd say the Weight -> Trotz coaching change has a much, much, much larger affect on defensive play than losing Tavares. Expecting Leddy and the rest of the Islanders to have a really good chance at "dropping" in value because Tavares plays for the Leafs is a silly and simplistic opinion to have.
I'd rather not trade Leddy for a "fair return". I'd rather keep him because his contract his great and he can hopefully mentor young kids like Toews, Dobson and Wilde. Contrary to what people lie about, Leddy has not asked for a trade nor is he an impending UFA anytime soon. I don't see a better defensemen available via trade right now, so meet our steep asking price or we'll just keep him.