I think you are only looking at on-ice factors, what I'm talking about is much more the off-ice factors, having veteran leadership and mentors, having someone that speaks the same language etc.
Most 1OA are good enough to be successful regardless. That's the whole point. Ovechkin's English was a wild mix of swear words and thick accent at first. He did score over 100pts in a season though. I know it's not fair to compare generational freaks of nature to an "average" 1OA, but just to put it in perspective. Of course there are numerous factors affecting development, but mostly for a 1OA they are not that important because of the talent level.
Kotkaniemi is not a great example, if he was #1OA he would have been heavily, heavily questioned.
Oh he is a good example. Exhibit A how if Dahlin was not a 1OA he would be questioned less. I.e. 1OA was not the right pick for Dahlin. That's exactly what I was saying.
Hughes and Pettersson have had some struggles this season and a lot of people have speculated it's linked to losing Markström/Tanev in the offseason.
I wouldn't say 24 points, tied for 2nd in the league among defencemen is exactly struggling. Yes, Hughes is not far ahead of Dahlin in +/-, but that again just depicts what I said before. Similar situations, bad team. But Hughes mostly fits the bill of a offensively gifted defeneceman who is yet to learn the other aspects of the game, which is not at all unusual. Dahlin is not only atrocious defensively, he is not scoring much. Three even strength points IIRC too. HE definitely looks more like a guy in trouble.
You're comparing to names without the same expectations on them, that didn't enter the NHL at 18 in many cases, that have had good/great veteran mentors around etc.
And again, that was my point and my logic. Dahlin taken not as a 1OA, but as a say top 10 pick would be beneficial for his development and the proper assertion of his talent. Less expectations, more patience, maybe a stint in the minors before the NHL pressure cooker. Right now it looks like he is not head and shoulders above those other prospects, but that was how he was presented to us at draft day.