Duly noted, and valid.
I think you do learn from an interview. This is true even if you work daily with both men. You can explicitly ask about and learn about their visions for the defense, what they would do differently, and what they would do the same. Also, part of the job of a position assistant is to implement the opinions of their boss and be good soldiers about it while they are on the clock. An interview lets them share more about who they really are rather than just giving Jon and Chan what they want.
Some people are natural captains and some people are natural lieutenants. Seeing people yelling and teaching on the practice field tell you something about their people skills and football knowledge, but not if they have the leadership to guide a defense.
There's a lot of Peter Principle in football.
Again. Again. Again. I am not in any way comparing the two men, with the one exception that I do think we would lose Giff before we lose BJ. And that does not mean I necessarily agree on the merits of that.
But if Tenuta moves on, and he just might after this year, we may lose everyone on the defensive side but who we promote anyway, and it may not be our first choice.