James is a world class defender when he tries to be. He does that in the playoffs these days, when his opponent is worthy.
Durant played spectacular defense in these playoffs, though.
Curry has never been a great defender, but he's been a league leader in steals the past few years, and when healthy manages his assignments pretty well. Far from a negative on that end.
I think you're right about the way that teams have learned to challenge the Warriors. That said, the reason the Warriors could not beat that strategy in the Finals was that Curry's knee injury severely limited his ability to attack switches off the dribble, as he did last year to such great effect. Just think about the last few minutes of the 4th quarter of Game 7 (I doubt I'll ever forget it ... ugh). A series of launches from three point land, including a pull-up from deep by Curry with Kevin Love switched onto him. The Warriors simply lost their mojo inside the arc. And yet, they were still a missed shot one way or another from winning the title.
Now, enter Durant. Durant has become a pretty devastating 1-on-1 player, in pretty much every situation, including attacking closeouts off the dribble. Far, far more dangerous than Harrison Barnes was. Barnes' inability to create off the dribble when the opposing defense keyed in heavily on his teammates was a major weakness for the Warriors in the playoffs. Especially, I reiterate, after Curry's injury.