Care to elaborate? Since football is built around schemes more than any other sport I'd say every position is open for inclusion depending on the system a particular team runs.
I love your explanation of HB, its perfect. It may even be enough for me to change my list. Seriously, no sarcasm.
However, I disagree about CB. If I have just an adequate CB I can still have a dominant defense provided I put consistent pressure on the QB (see the last 2 SB champions) and/or otherwise hide them with zone schemes or help over top. Besides, there are so few really good ones to be had. Perhaps this is why the best get paid so much and teams are constantly shelling out big bucks to fill the spot.
Its not as difficult to cover up a MLB/Safety/Guard. Its almost impossible to mask a bad CB/LT/QB in contrast you can exploit it all day no matter what else is going on.
A lack of good RB or PassRusher puts immediate and immense pressure on the other aspects to be very good.
There are no hard and fast rules. People seem to be in love with naming random team who got by without such and such position. That doesnt prove anyones point.
Parcells , Jimmy Johnson , Walsh are all going to weight certain positions more than others. But by and large the positions I mentioned are the ones most key to winning and losing any given football game. They are the position most paid attention to in all aspects of team building.
You put together 5 solid to all pro guys at those positions youre better off than having 5 solid to all pro guys at any other position.
On the flip side you can have all pro talent everywhere else but if you have doo doo at those spots youre not winning much of anything.
As for CB. As mentioned the scarcity in talent and rule changes makes it hard to put my finger on. Its just the hardest position to cover up on defense. Like LT on offense your on an island and your dependence on your teammates is less than anywhere else. With either spot your essentially taking someone out of there normal position to help you. MLB for example can be covered easily by decent/good DT play. A great MLB makes alot of plays but doesnt necessarily free up anyone else or stop anything in particular. Things still need to go AT him. A top tier CB
immediately takes away a primary target
and frees up an additional player ,regardless of pass rush or if anyone goes that way.
You
have to have exceptional pass rush to mask it. On top of very competent safeties.