NHL teams appear to be better at drafting defensemen these days than even five years ago. Basically, if you're a CHL defenseman who is not putting up around .7 or .8 points a game, you arent going to sniff the top 20, maybe not even the first round. This wasn't always the case, as mediocre defenseman like Gudbranson, Schenn, Murray were all drafted very high, not to mention Dylan McIlrath who was drafted 10th overall, but didn't project to be anything more than a better version of John Scott.
PPG in junior (or also for the US developmental team) for a defenseman isn't a perfect metric, but it's a lot better than what teams seemed to rely on in the past. I imagine that defenseman drafted in the first 15 picks now have a higher success rate than they used to, and they make a difference at the NHL level sooner, and almost at the same rate as forwards drafted in the same range.
It seems to me that the traditional makeup of a defender is changing. Previously size and strength were primary with skating and skill behind with the exception of obvious great players like Potvin or Bourque where you had the whole package. Now teams seem to be leaning much heavier on skating and skill with size and toughness behind. I think we will see players get a bit smaller but they are getting much faster on average and defenders have to speed up and be more skilled as well. It's not a perfect metric but generally the more skilled players score the most points.