First because very few players become first liners. Second, usually those guys display dominance from a very early age. Butcher displayed quality play, but not dominance. Stepan looked like a man among boys in the WJC. Butcher looked like a nice player. When he was a year younger than Butcher and had no pro experience, Stepan left no doubt during the preseason that he'll immediately be a key player - and then scored 3 goals plus a goal post in his first game. Then Step developed decently well, not like he suddenly became a bust.
And even Step is only a borderline first liner, not a superstar. One of the main reasons people give for us not winning the Cup is that we did not have a dominant 1C, just an ok one. What reason do we have to believe that Butcher will go from a lower point right now (than Step at the same age) to at least an equal point? I know it's just one player, but it's a pretty representative example. Becoming a first liner is very, very hard. Usually the talent to do so is obvious by the time you're 21.5 years old.
There's nothing wrong with being a good second liner. Nobody complains about Miller and Kreider, and the team gave up first rounders to get him. If a third rounder Butcher becomes their equal (in value, not style), it's an excellent result. Expecting him to become a 70-point player is a bit optimistic.
Very few players becoming first liners is a bad argument. We aren't talking about some random player, we are talking about a very good NHL prospect, so the odds should go from nearly 0 to a significant percentage. Lets just say conservatively, almost everyone has him ranked as a top 40 prospect, some have him ranked him than that, how many of those top 40 NHL prospects don't have significant chances to be first liners/top pairing defenseman?
Plenty of players have bad WJC's and become very good players, and plenty have very good WJC's and don't become NHL'ers. Buch had 13 points in 14 WJC games over two tournaments. For a top prospect, not great, but not terrible. If you go back and check those that became NHL first liners, I doubt that many of them were much better than right around 1PPG over 10+ games. I think its a mistake to think that type of WJC performance either qualifies or disqualifies him from being a 1st liner in the NHL. And if we are talking about international tournaments, why are you only counting the WJC-20's? Why not the WJC-18's? Buch had the second most points in that tournament his draft year behind some guy named McDavid.
What mainly qualifies a player is usually how they play with their professional team, college team, junior team, not a few international games. If you go purely on KHL stats from ages 18-21, he's only worse than Kuznetsov and Tarasenko, and better than Panarin, so if you are using those examples, it would make sense that he would become a first liner. We can all guess at what his current level compared to the NHL is, but he's played two preseason games, and he's certainly not been out of place, one might even say he's been very good. Would Stepan have had 6 points in those two preseason games? I know so many people have unrealistic expectations, but he doesn't need to be the best player on the team in preseason to prove that he's a top 9 NHL player this season.
His skill-set compares favorably to second liners on the Rangers like Kreider and Miller. If we are grading on a 20-80 scale, he projects in the future as at least a 50 grade player in every single offensive category, whether its shot, hands, skating, puck protection, puck handling, playmaking, etc. Compare that to players like Kreider or Miller who although have some very nice tools, have clear weaknesses in their offensive games. If I was to guess, I think he'll be around a Stepan level NHL player, and to be clear, I think Stepan is a first liner, if the qualification of a first line center is the top 30 centers in the NHL, instead of just some arbitrary cut-off where people feel comfortable saying a player is a 1C.
