First thing - obviously Boucher isn't your typical 10th overall. I look at the 2019 draft, where apparently Lassi Thomson was 19th on Ottawa's list and they made him the 19th selection. If they have pick 18, and Boucher and Lassi are on the table (we need to use our imaginations a bit here) I am not sure they take either guy over Harley. Almost certainly at 17 they would have taken Krebs. So I think Boucher needs to be viewed in that context - for better or worse, the Sens didn't love what they could get after 9 in this draft, and so they took a guy who they were confident would fill a need down the road. I think most people don't love that as an approach to drafting, but I do think its a reality of the situation and one that should be acknowledged if we are to have a clear picture of what went into the selection.
Secondly, there are plenty of top 15 pick FWDs who have taken 4-5yrs and turned out just fine. Blake Wheeler didn't play in the NHL until 5 seasons post-draft; Grabner wasn't a full-time NHLer until 5-years post-draft; Couture took 4-yrs; Eller took 4-yrs; Kadri took 5 years; Kassian took 5 years; Nino took 4-yrs; Schwartz took 4-yrs; R. Strome took 4yrs; JT Miller took 5yrs; I could go on.
Also, no one expects him to dominate offensively in the WJHC and no one said that - the hope is that he will be a presence (and, of course, be present!)