I understand where you are coming from, but other than Rathbone, none of those guys were impact players right after coming to North America. Rathbone sustained a gnarly injury which hampered his development and he hasn't been the same since. Pretty much all of those players listed either didn't join the AHL in their draft +3 years or they never even became impact players in the NA to begin with.
That is what seperates Pettersson and Kudryavstev from those players, they are a year ahead of where they are expected to be. It is why I think you should be excited at where they are at in their development.
Further to that, one is allowed to be pleased and even excited by their development without that meaning one thinks they're very likely to succeed at the next level.
Kudryavtsev and Pettersson are 7th and 8th in points for rookie defensemen in the AHL. Above them there's one first round pick, three seconds, a third and a fourth. The top three are all undersized, offense-first guys getting power play opportunities. The rest are closely bunched.
I think that EP ranks second in even strength points.
Looking at their games played and their production, KK and D-Petey compare quite well to the nine defensemen taken in the first round of the same draft year, with most of them also playing in the A, and a few getting some games in the NHL on non-playoff teams.
Obviously point production is a very partial indicator, and the sample size still isn't very big, but that's pretty good for a third and a seventh rounder.