I am a bit surprised that no one has chimed in with the obvious, so I will:

There are many who complain that Trump/Sessions should have fired Rosenstein a long time ago. So why hasn't he?

I believe the hint you need is in the OP's very well written article - Rosenstein was instrumental in getting Comey out of the way. "Comey gets fired based upon a letter by Rosenstein."

So, at least there is a possibility that Rosenstein didn't just do this to move up the corporate ladder. Maybe there is something deeper?

Try to think like a prosecutor. Your ace in the hole is when you have a surprise witness turn state's evidence.

What if Sessions got Barsoomian to flip?

Image the damage that one person could do.