In reality the current Suns aren't that much worse defensively than in the last five years or so. Statistically they are worse, but a large part of it is due to less efficient and much less potent offense. In the past, the Suns could get away with bad defense more because offensively they could not only outscore most opponents but also flat out demoralize them. Those past Suns teams would score at will, on misses and makes, score quick, inside and out, to the point that opponents often couldn't get into a good rhythm offensively. The Suns had an answer for every hoop, for every run, and could erase a double-digit deficit in a few possessions.
This is no longer the case. The Suns can still put up points, but not the same kind of points, and not in the same manner. Obviously the biggest reason for that is a lack of dominant inside scorer that we had in Amare. Amare might have not done much on the defensive end, but the offensive punch that he and Nash provided together was enough to offset some of the team's defensive shortcomings.
The current Suns squad needs to realize that they can no longer beat anyone just by outscoring them. The 76ers game could have easily been a win in previous seasons even when the Suns gave up 120, because the 76ers played poor defense as well and the old Suns would have scored 130. In the past, defense was what the Suns needed to be a title contender. Now, it's what they absolutely must have just to prevent being a sub-.500 team. The team also needs to realize that it is not enough to talk about defense, and not enough to practice it. I remember how D'Antoni was blamed by many here for lack of defensive emphasis. Now we have all the emphasis in the world and still no results. The players must learn to execute what they practice. They are already learning that they cannot win games otherwise.