Let's forget that Amare was actually drafted as a defender first and that in fewer minutes and fewer possessions as a rookie averaged as many rebounds per game as he was ever averaged under the increased possession D'Antoni system.
Let's also forget that the Suns have had the ability to play quality defensive centers when they've had them, but didn't, whether it be Hunter, Thomas or Skinner, each of whom averaged roughly 12-to-16 minutes per game.
Let's also forget that under D'Antoni's fabulous GMing, the Suns traded multiple picks, one that could have net them Rondo.
Part of the reason the Suns defense was always so pourus, was it was sacrificed to maximize offensive potential, meaning going small to gain a speed/shooting advantage and the like. This stemmed from D'Antoni's system.
Finally, to enable his 7-man rotations, D'Antoni frowns upon physical defense. Such often can lead to fouls and FTs. The former gets his thin rotation in foul trouble, forcing him to extend to players he hasn't chosen to coach, while the latter slows action and allows defenders to get back on defense.
I'm telling you, everything that's wrong stems from D'Antoni coaching, his system or his GM failings.