I think it's a talent driven league. Sean Lee is an elite player. Lawrence looked elite this last season. Hopefully he stays healthy and pees clean. The rest are JAGs and young guys. You seemingly dismissed the rookies because they didn't have the most snaps, but by season's end they were starting and the defense was better.
You're basically saying they should have a top defense with what they have by saying another DC could get more out of them. They had no true 1 tech. Their interior defensive line is weak against the run. Jaylon had to learn football again. Hitchens is too small to be a consistently effective Mike and too slow to be a consistently effective Will. Byron Jones has no ball skills. How does a DC fix those issues?
The defense was "better" at the end of the season partially because who they were playing, not solely because they got better. They faced a Giants team that was woeful offensively and faced an Eagles team that benched all their starters and weren't a good offense even with Foles at QB.
Your last paragraph is excuse making and actually undermines your argument. Why did we have an issue at the 1 Tech? I'll tell you why......... because our DC doesn't value 1 Techs. He thinks he can find any joe blow off the street and make them a 1 Tech. It's why over the years under him, we've invested zero premium assets in a 1 Tech. Hayden, McClain, Paea, Thornton, etc. Those are all guys Marinelli wanted. So to sit there and try to say, "poor Marinelli, he didn't have a real 1 Tech" is disingenuous because the reason we don't have a true 1 Tech is because of choices Marinelli has made at the position.
And who makes the decision to play Jones at FS as much as he does? Who made the decision to eschew improving the S position in FA last year and give Heath the job? Marinelli. That's who. You are trying to make excuses for talent and personnel issues and yet many of the personnel issues are tied to Marinelli himself.
Again, nowhere have I suggested that he should make this into a top defense. But for sure, I expect a more consistent defense out of him, especially one that doesn't turn into one of the worst defenses in the league because one player is out.