It's sort of a double hindsight argument.
Six weeks ago, we were feeling pretty good about Howell and the offense was doing relatively better. We were frustrated at the lack of a running game and thought the blocking was killing us, but Bienemy was holding us in games despite JDR and Romeny. When the catastrophic games happened, getting obliterated by the Bears and the Giants, a number of people thought Ron should be fired and the question of whether Bienemy would ever get his shot was still in the air. The argument against Bienemy was actually that the offense and Howell was doing pretty well and did we want to change that which could result in Howell not receiving the guidance he needs.
Fast forward, we now have benched Howell and we lost the opportunity to see if Bienemy had the ability to become our head coach. I do agree with you that given the way the offense fell apart (though it looked damn good with Brissett which could be a mirage) that he probably wouldn't have executed the pivot and shown himself to be a great head coach prospect for the future. Still, it might have been interesting to see how he would have handled it. Clearly, the team checked out on Ron defensively pretty much from the onset of the season and now the offense looks like it's cratering too so it may not have changed much, but it would have given us a new data point.