I have a couple of arguments with David’s piece:

The issue is how to exert pressure (which he says we mostly can’t)….

Actually, we can: ideologically, economically, and even militarily.

1) Stepped up propaganda in the form of Radio Free Europe and Radio Liberty-style broadcasts into Iran would provide moral support to Iranians who oppose the regime. (I have the impression, perhaps wrong, that our efforts so far are generally minimal and half-hearted.)

2) Iran is incredibly vulnerable to a cutoff in fuel supplies, given their reliance on imports of gasoline. It would cause havoc in Western economies but, if deemed necessary, I think it would cripple the regime quickly. Also, I don’t think we can underestimate the effect of financial sanctions that hit the black-robed mafiosi where it hurts — in their bank accounts.

3) Disaffected subject peoples within Iran have already been striking back, attacking government security forces in the Kurdish and (IIRC) Khuzestan Arab regions. Iran’s been killing our people, and I suspect we’re providing support to anti-government forces in their area. We can always ramp that up.

Then:
Opinion is turning slowly against extremism.

I have to take issue with David and the other experts, here. While Zawahiri’s old ally may have turned against violent jihad, it is the “violence now” part he and others are rejecting, not the doctrines of Islamic supremacism or the pursuit of “cultural jihad” through the exploitation of Western liberalism. It’s an argument about tactics amongst revolutionaries, not a renunciation of the revolution.