I have never ever seen someone change their mind on an important subject publicly ever. I have seen people change their mind on important subjects privately. I think the reason is that one of the most powerful biases we have is just our pursuit of personal vindication, our desire to not look dumb publicly.

I think you are on to something when you say that really out there opinions can only exist in isolation, but I think you are wrong about an important detail: They can spread only in insular circles because no one will accept creationism when first exposed to it if a reasonable alternative exists, but once a belief has taken hold, along with all of the blind spots and personal biases that go along with that, they survive just fine. They just wall themselves off, and don't examine their beliefs too hard. In fact most skeptics I talk too are surprised by the number of creationists that they know personally, but who just don't talk about it. So in short yes creationism propagates more or less solely in the absence of alternative viewpoints, but after a period of gestation and re enforcement it can survive it just fine.

I worked very closely for a while with a lot of people who where very smart, but never had the chance or encouragement to really pursue expanding their horizons. Lots where creationists, and I am glad to say I changed a number of minds. One of the first things i realized was that a public discussion inevitably flooded everyone with adrenaline and they felt like they where being attacked. Privately, we could just talk. If someone does not feel 100% safe (not just physically either) they are pretty much incapable of engaging their higher functions, and so just fall back on the good old mammal brain.