Talent reasons alone - I think there's an argument to bench him because his accuracy is bad, his decision making is questionable, and he really looks like a RB trying to play QB in a passing offense - and that has never worked for any team.
But, I think the biggest reason for Riley to consider a benching is due to that 3rd down play. If Mike Riley and the OC really called a Bootleg Run with no passing option and Armstrong freelanced himself into a pass - then to me that is simply insubordination by your QB. You cant have your QB not running the plays that are called on any football team. Everyone has to be on the same page. And this isnt high school football where a kid makes a mistake and its ok we'll get em the next week. Its not. This is big time D1 football and guy's careers are on the line.
IMO, I think Riley and his staff should use yesterday as a teaching moment. He should hold Armstrong accountable for his actions and at least bench him for the 1st half of the next game and maybe even the whole game depending on how the backup plays. He needs to make it clear that freelancing is not acceptable. That playing smart football is 100% necessary at all times. And that blatant intentional willful insubordination will not be tolerated. The QB runs the plays that are called period.
There's too much at stake here and if I was the coach, I'd have a real problem trusting Armstrong from here on out. If he cant even run the simplest plays correctly, how can he be trusted for other things? I wouldnt allow him to make that mistake again. I'd try the next guy.