What are the chances that any skater we pick in the mid teens will become an impact player in the NHL?
Pretty decent, if you ask history. About the same chance as a goalie would have. Riku Helenius, Chet Pickard, and Jack Campbell were selected in that range, so it's not like Knight is a 100% guaranteed NHL starter just because he's ranked high.
You just can't be certain with goalies, because SO MUCH has to happen between their draft year and becoming an NHL starter 4-6 years later. Picking one high and forgetting it isn't wise, because of all the development that has to happen in the intervening years. They're so far away from NHL action that anything can happen in between.
I'm not a huge fan of Hepple's Euro overager strategy, but it's not like spending a bunch of high picks on goalies is a proven strategy either. They bet pretty big on Annunen last year, who was a consensus highly ranked tender, so maybe that's changing, but I don't think they would spend a first on a goalie just because we have goalie issues right now and consider the problem fixed.