Franks would do better to point out the problem with the authorization rather than calling for Mueller to resign. The first has fact on its side, the second looks partisan.
Pound the facts. And bring pressure on Rosenstein to withdraw the authorization.
I agree. He would also do better to make his argument "global" instead of depending on Mueller talking a Trump obstruction (fired Comey) investigation, a premise that as far as I know, does not exist. But if it did, Mueller can cure it by abandoning ONLY that.