I gotta disagree with you here on a couple of points. First, Pryor's a junior...he's not leaving. And he's come a long way this year IMO. And Hegs and Nemec have been shutting down the top lines for the top teams in the country for most of the season. Pairing them with Mangene's line against the Kreider line vs. BC. and the Chiasson line against BU worked great. Not always pretty, but effective. And Hegs is getting chances when they put him up front on the PP....he just needs to start burying them. As far as Nemec, he might not be here next year (graduates this year even though he has a year of eligibility left). Without those two, we don't have a shutdown D.
I think we'll still have a solid top 4 next year with Cornell, O'Connor, Rutt, and Pryor...well at least top 3. It gets pretty scary after that. And the biggest addition by subtraction is definitely O'Neill. You only have to look at his +/- vs. his D partner, Cornell to see that he's a huge liability. I think he has been out there (and arguably responsible for) all 9 -- yes, 9 -- shorties against this year. And to be skating with the top line in the country on even strength and have a negative +/- is astounding. Watch how much better Cornell is when he's not covering for O'Neill.
And this D crew is playing well in front of average goaltending. Let's not forget that Sully is still in he bottom half of the country statistically. Far better than it was last year and early this year, but far from lights out.