Lets say the opponent runs out of an i-form set. You have potentially 7 blockers on the play: All 5 OL, TE, FB. Even if all 3 linemen ate up 2 blockers (which is unrealistic), you still are going to need the initial linebacker to shed a block.
It's unrealistic to think that each member of a 3-4 DLine is going to take up two blockers on each play. Ideally, what you're looking for is 3 players who will eat up all five offensive linemen, and middle linebackers with the ability to avoid & shed blocks from TEs, FBs, etc, to be able to make tackles.
That's how the Steeler defense works anyway.