Quite true.Suh was probably a once in a lifetime player. I don't remember anyone being that good when I watched all the tapes of '95 and '97 when we had amazing defenses.
Assuming somebody will just 'step in' to Suh's shoes is a pretty poor assessment. Somebody will fill the gap well, yes, but they will not be a player at the level that Suh is at.
No one should expect suh.
That said, I'd guess that Crick will be at, or near, all conference and that Steinkuhler will be nearly as good as Crick. That 1-2 punch in the middle will be really quite strong. I'd guess that Allen will be better next year than he was this year. So too for Crick. I'd guess that Cameron Meredith will be just as good, if not slightly better, than Turner. So... I see 3 of the 4 positions on the D-Line improving in terms of starters. And I'd guess the depth will be better across the board. The only position that will be a downgrade --- which by definition it must be --- is replacing Suh. Still.... Steinkuhler will be really, really good (not Suh good ... but really good nonetheless).
So.... I am thinking that the D-line will be as good (or close to as good) next year as they were this year. And ... the back 7 will be better next year than this year. Overall? The defense improves.