Not sure where to put this, but I spent a few minutes looking into William Karlsson's analytics. There's nothing in his NHL numbers that could have predicted his 43 goal season this year. All of his previous season's numbers and underlying analytics were average/below average. Anything that above average, were you to base a prediction of 40+ goals on it, would end up looking very silly. The only thing that I could find that was exceptional about him was what he did before he came to the NHL.
1.) 7th all time U18 point production in SuperElit (4th is Berggren btw)
2.) 2nd all time U19 point production in Allsvenskan
3.) Led his SHL team in scoring as a 21 year old (37 points, same year Arvidsson had 40)
Interestingly enough, point number 2 includes Alexander Wennberg. Now, this is not to say that every player that puts up historically good numbers within their age cohort in a pro-league will go on to be a 40 goal scorer. But the William Karlsson story is, IMO, a case of an offensively talented player getting to play big minutes with other offensively talented players, and nothing more.
Why he never got the chance to Columbus to strut his stuff I'll never know. But the reason why they had no idea what they had is pretty clear: they buried him, played him with 20-30 pt players with 16 minutes total PP time for the entire season.