To say that Juolevi has regressed or hasn't improved is absurd. Especially looking at his statistics but not looking at the context. The context here being #1) Tkachuk, Marner, and Dvorak graduated into the NHL, and #2) Juolevi is used primarily as the main shutdown guy.
Last season, the London Knights scored 319 goals as a team. Juolevi was in on 42 of those goals, meaning that he contributed 13.17% of his team's total offense. This year the Knights scored 289 goals as a team and Juolevi was in on 42 of them. He contributed 14.53% of his team's total offense. We can see clearly that -- despite Tkachuk, Marner, and Dvorak graduating into the NHL -- Juolevi's offense did improve. The increase in percentage points doesn't look like much, but keep in mind if Juolevi was in on 8 more goals (50 points total for the 2016.17 season) he would have contributed 17.30% of his team's total offense.
IIRC, Juolevi was used primarily as the main shutdown guy. The most recent example of this was during the OHL playoffs, where he was consistently matched up against Gabe Vilardi. That series the London Knights eliminated the Windsor Spitfires from the OHL playoffs and Juolevi and co. held Vilardi to 6 points (2 goals and 4 assists) in 7 games, with a plus 1 rating. Currently, at the Memorial Cup Vilardi has exploded offensively in his most recent game. In 3 games Vilardi has 5 points (0 goals and 5 assists) with a plus 3 rating. Small sample size I know but Juolevi was a beast defensively in his OHL Playoff series against Vilardi and the Spitfires.
tl;dr Considering the context of his season, Juolevi was put into a more defensive role handling top shutdown duties this season yet still managed to increase his offensive contribution; Juolevi had a good developmental season.
Getting back to the other draft prospects... I have no idea who I want. There's so many good prospects where we are. Glass, Vilardi, Mittelstadt, and even Necas or Pettersson have an outside shot of sneaking into the top-5. On the defensive end of things Heiskanen might be the top defenseman but don't sleep on Liljegren because of his mono and Makar because of him playing in the AJHL. I hope Benning can get another pick in the top-10 so we can draft a playmaking centre and a puck-moving pp quarterback defenseman. Make the rebuild go by that much faster.