This is the direction I'm heading in, but without QB at the top. I used to feel d-line was more important than secondary, but now I feel the opposite. Linebackers still rank last in my grading of unit importance on the defense.
Every scheme is different, but I generally agree with you about linebackers. In passing situations, one or two linebackers are pulled for additional DBs; in running situations, one or two linebackers are pulled for additional DLs. So linebackers as a group often don't have a big enough role in the defense to warrant being the highest paid guys on D. (Though there are exceptions.)I think QB wins because the whole offense goes through him. Passing, obviously, but he also sets up the run with the threat to pass.
It's telling that people point to individual examples of Super Bowl teams that didn't have great QBs (Trent Dilfer being the most recent), but nobody points to individual examples of Super Bowl teams that didn't have a great left OG, or strong safety, or tight end, etc. Because those things aren't anomalous; they're not worth pointing out. A great team with a below-average QB, however, is anomalous.
After QB, I think left tackle is most important. He protects the QB's blind side, and a good LT makes a huge difference in the overall gameplan for the offense. If the LT can lock up with the opposing team's best pass-rusher one on one, that leaves the RB or TE free to release into the pattern and creates matchup problems for the defense. If the LT needs constant help from another blocker, the offense is at a disadvantage with respect to the defense over the rest of the field.
Same sort of reasoning explains the importance of CBs. The two most important jobs the defense has are (1) stopping the pass, and (2) stopping the run. It's obvious how having top-notch CBs can help stop the pass, so I won't comment further on that. But having two corners who can play man-to-man without help is also the key to having a great run-stuffing defense, as well as generating a good pass-rush.
If you have two CBs who can hold their own in man coverage (think Mike Haynes and Lester Hayes on the old Raider teams), that gives the defense tremendous flexibility on how they can use the other nine players. They can put extra people in the box to stop the run, or blitz extra people to generate a pass rush. Even if they are outmanned, talentwise, in the front seven, an extra body up there more than makes up for it.
So aside from QB, I'd list LT, CB, and CB in that order as being the most important positions.