Based on a metric of producing code that fulfills the requirements, they may be as much an A player as the assertive developer down the hall. The thought of including assertiveness in measuring employee performance has never occurred to them. You say to them that only "A" players hire equally capable people, but they respond by saying that they are an "A" player. It is the start of an unreasonable argument.

Perhaps they should hire for assertiveness, but that's a matter of convincing people that their method of analysis is wrong. That doesn't get solved with platitudes; it gets solved by showing that encouraging assertiveness provide more value that the security of the status quo. How to show this, though, is something I am still trying to figure out.