There are a lot of very talented prospects that have not made the jump to the NHL this year. Some were expected to, some were not.
However, to state that Nylander could do what Ehlers is doing, is just false. Ehlers is a phenomenal back checker that uses his speed to his advantage, and has above average hockey IQ (according to those in the know). Plus, he still brings that element of elite offensive talent.
What makes you think William is that same player? The minor leagues are littered with players oozing with offensive talent. But can they play the 'game' at an NHL level?
As a side note: Sean Couturier (8th overall/2011) made the jump before Mark Scheifele (7th overall/2011). So what!! Looking back, I think that extra year helped Mark, and I wouldn't change that pick for anything.