You're being intentionally thick and misleading.

The 5th DB is extremely important and is much clearer than "3rd DL" as DL in general are much more specialized and situational than most DB's. Yes the LB are the most important unit on the defense because they have the edge rushers. Its super common to draft a college DE and convert them to OLB in a 3-4, in fact that's where most of them come from. They are edge rushers first and foremost.

Try this logic - if you're advocating a 3-3 but think one of our LB in a 2-4 is a DL anyways aren't you saying the solution to our defense is the difference between a guy standing or not?