It is like Broaddus wants to start trouble via his Twitter followers.
In football (and especially at the professional level), team rosters are usually built with the relative importance of specific positions--Quarterback, Left Offensive Tackle and passer rusher (usually Defensive End)--occupying the top three. Those three are sometimes interchangeable, with Left Tackle sometimes being the foundation over Quarterback, etc.
Through the years, Dallas has employed an offensive scheme with an outstanding rusher as its center piece. Emmitt Smith is the perfect example of the 90's teams. Slightly less so was Tony Dorsett by comparison but there is no mistaking how valuable Dorsett was from his very first year as a Cowboy--as in Super Bowl caliber Running Back. Ezekiel Elliott fits the mold of Emmitt Smith in Garrett's scheme and demonstrated his outstanding worth during his rookie season.
The answer to Broaddus question in my opinion? I would say there are two answers.
Commonly league wide, the answer would be Running Back. However, the importance of a superb rusher in Dallas' offensive scheme elevates that position as a top three mandate. Left Tackle is never out of the top three for any reason. The remaining two positions are Quarterback and Defensive End.
Traditionally, consistent efensive pressure can be generated from Defensive Tackle or Outside Linebacker if either or both positions or can be filled with great candidates. There is flexibility there but none concerning Quarterback. Whether Quarterback is drafted or traded for, he is the orchestrator of the team's offense. No exception. Demarcus Lawrence would be the logical sacrifice given all considerations.