I wouldn't bank on Mangum leaving this year. There is no way he can repeat the numbers of last year, just not practical, plus he'll still have leverage after this year. While he may not bat over .400 he'll provide the necessary leadership and all the other intangibles.
Blake Smith and Brent Rooker are about the only sure fire guys we will lose to the draft, IMO. I don't know how Ryan Rigby projects, but if he's drafted highly enough to sign, that probably means we had a great year. I wish we could also lose a few of the JUCOs to the draft too, that means we had an even better year.
Anyways, I think in 2018 we can overcome most of that.