...that there isn't an 'Iraq' that can exist in any meaningful form post-withdrawal, without changes so large in both government and borders that it's rendered unrecognisable.

I can foresee a future where we get out, under whatever circumstances, and Iraq more-or-less immediately plunges into civil war, the outcome of which will be an independent Kurdistan (which may, if Turkey can be reined in, be the stable touchstone in the region) and two Arab states, one Sunni and one Shia.

The wildcards are, of course, the various Islamic neighbour states, who will probably cast their lot with Sunni or Shia accordingly.

Iran will probably take the opportunity to expand its border to accommodate ethnic Persians, too.