A partition would be tempting, but would result in a) massive ethnic cleansing b) a new Iran client state and c) a completely lawless Sunni state.
We already have those things. At least a partition could possibly bring some form of stability to the area.
Yes, but it would codify them. At least ISIS should we wiped out before we start talking about partitioning.
Well we cleansed Anbar of the bad guys once, and left, and see what happened? So what now? The Shia militias go in with US help, and commit their own atrocities, or do we just try to contain ISIS, while they kill of all their opposition (e.g. the tribal leaders), or does the US does it all over again at the cost of more lives and money, and if the latter option, after the US does it all over again, then what? Maybe the US should demand that Iraq partition itself effectively, before the US does anything at all, if then.
I really think at this point the US has no choice but to recognize the bad guys will control real estate in the region (of many different hues), and we are left with the rather unglamorous policy of containment, and selective bombing, and so forth. There are just too many crazies around there, to do anything else.