I doubt Nill did nothing on D for lack of trying. Please tell us what the options were if you thought he should have made a deal.
Forumites are certainly panning any reasonable option he did have, like Dion P. Yes, he should have been in on Boychuck, but we don't know that he wasn't. Nor were we sure Boychuck would be a true number one guy, either. I think everyone was shooting even higher, and when that wasn't attainable, he thought maybe letting the young guys play was as good an option as any for long term improvement. BTW, I happen to agree. They did make necessary, unavoidable steps forward that any young D must make and struggle to make.
You may say he should be gone for missing playoffs year two, but Nill, TG, and the whole organization made it clear this was a long term process not to be measured in one or two years of individual seasons.
There will probably be a fall guy, just to show some changes, but it would probably be a guy like Valley over a bigger name or position. And, if so, that sounds like about the right move for the franchise in its infancy of building something long term.