The way I see it, if you are certain, you feel like you know for sure that this guy is going to be able to play 30 minutes a game for you, and whenever he's on the ice you feel safe and confident that he will make the right play to keep the puck out of his own net most of the time, sometimes even help put the puck in their net, absolutely worth a 1st overall pick.
If you go ask any GM in the league, maybe not every single one but at least most, would you rather have a 1D, a 1C, or a 1W, the answer is a 1D every time because the 1D can play up to half the game. There's a great irony of saying "well what if he ends up like Colton Parayko", well, Colton Parayko logged the most ice time in the Stanley Cup finals when the Blues won the Stanley Cup and became league champions. And there are many top scoring "high ceiling" players out there who haven't sniffed a cup, much less led their teams to one. Not to mention of course, his top 4 roles with over 20 ATOI in the 2016 Semifinal and 2017 Second Round runs. So if I'm a GM and the WORST case scenario for this player I'm about to draft is that he can lead my skaters in ice time to a cup win...I'm pretty ok with that.