Of course it’s possible Iraq would’ve gone different without the Coalition Provisional Authority’s early orders.
If the last several dictators ousted from nations in the middle east teaches us anything it's that we do not want the hydra. It cost trillions of dollars and thousands of lives to dump all the crap out of the Iraq vacuum. Egypt was teetering on the edge for quite awhile after the arab spring. Libya is still in ruins. Syria speaks for itself. Afghanistan is slipping back into Taliban control. Every time we touch it it gets worse.
While I think we can all agree that totalitarian rulers are not the global maximum they seem to be a pretty clear local maximum in the middle east. The situations that develop in their absence are so much worse on a "suffering per time" basis that it's basically impossible to justify trying to oust them.
I believe you're right on the money with your point about a local maximum.
Even a bad ruler is better than anomie, especially for local minorities. The yazidis may not have been happy with Assad or Saddam, but I'm pretty sure they preferred them to ISIS and similar groups.
Also, "unwavering ally" and "we'll remove their state and let the country fall into civil war" aren't the only options on the table.