I'll be happily surprised if Johnson comes back; he's already been impressing NFL people to the point I think he'd go by the end of the second round. I wouldn't call Mangiro a liability, but I don't think there will be much of a drop off to Laurent, based on what little we've seen of him. Next year and beyond, we have McGovern on board, who is thought highly of as a center. The other four starting OL will be back and - we hope - stronger and better, we have a lot of young (and really BIG!) talent already on board, and some very good ones coming in.
I agree that Lucas will be a loss, but Apke is getting some quality reps, and his speed is an upgrade from Lucas. I just hope his "football speed" catches up, which is where Lucas shines. As long as Allen fully recovers from whatever put him in a sling against SDSU, we should be okay at safety. Remember we also have Petrishen, Farmer and Oruwariye waiting in the wings, Taylor/Monroe/Miller red-shirting as true freshmen, and Pryts incoming.
I agree with you that losing Williams will not hurt too much, especially with Reid getting a ton of playing time. We also have Haley and Campbell for a couple more years, with Hill (I have faith - or hope - that he won't flip) and McPhearson (CB or S) incoming.
Losing Zettel, Nassib, and (probably, IMO) Johnson will be felt more than anything, but the guys behind them have looked pretty good, even when they come in as a whole replacement unit. We have Sickels back, and I think he'll be much improved with a year as a starter under his belt. Also at DE we have Brown, Cothran and Schwan who are all seeing the field quite a bit, Buchholz, Carter and Miller as true freshmen, and Simmons and Dioubate coming in. At DT we've got Cothren and White coming back and Colon and Jordan coming in.
I don't know about people thinking last year would be the nadir for this team. I'd mostly heard 2017 or even 2018 would be the lowest, considering the sanctions would be in effect through 2016, and it would take time to recover. That was modified to 2015 or 2016 once the sanctions were lifted early. I'm not expecting a championship next year, especially with probably having a new starting QB, but I think we'll be better overall than we are now. I believe we'll be competitive with just about anybody in 2017.