If you had to ask me to choose whether to fire beane or McDermott, I choose beane. Why? While he's overall built a competitive roster, he's utterly failed at drafting gane-changing players.
You know my position on some of his picks, but ask yourself honestly: would you rather have a Metcalf or an Oliver? Basham or Humphrey? AJE or Uche? I could go on, but beane seems to prefer a whole roster of average over high ceiling guys (oddly, with the exception of the QB).
Also, this team lacks speed all over the field on both offense and defense, and I'd argue that the lack of speed on defense is the most notable reason why this team has failed in the postseason.
Lastly, beane's cap management and free agent acquisitions have been suboptimal, and the cap issues are going to do us in this coming season, I get the feeling. So, if you're asking me who's holding the bag of failure, it's Brandon Beane.