Answere from an old knuckle-dragging guy who spent 3 decades on defense...well a bit of an answer:
On tackling, that's mostly a CBA thing in the NFL, The NFLPA has successfully removed most of the tackling drills and padded practices from the table. Off-season workouts are 98% shorts and helmets. Who you "hire" has very little bearing on this because they are coming in half &$$ ed from college in terms of tackling and there is no way for the coaching staff to hone those skills anymore. I hate it too but that's what we got.
On the 46 and some of the zone stuff, again the era we live in. We sat on here for 5-plus years talking about the spread O and how it would change things in terms of the scheme, and so that kind of defense is easily defeated by the Chiefs of the world. Thus you see teams having to defend the way they do. Add in if I want to defend man press all the time I literally need 3-4 man press corners on the field and 2 in reserve.... so now you're getting into salary cap and personal issues. It's a very complicated thing really.
That's where your adapt during the year goes out the window, you're limited by cap and roster rules on what you can and can't do at the NFL level.
I hear you loud and clear, but it's just not that simple anymore.
