Abandon, driven off, however you want to phrase it. The writers deliberately worded it in a way that did not imply that followers would be subsumed within the believers, but that they would leave. Given that followers are generally depicted as above-average but not extraordinary folk outside the typical comforts of hearth and home, it would stand that their wills would not be as easily sublimated as the farming folk and city trash that turn up as believers.
And no, I'm not assuming that they have the Leadership feat. You're the one arguing the tone of the writing, and I'm using what's written. Also, if we ignore balance game mechanics, we throw out all the noise you made on it being Extraordinary rather than Supernatural. Either way, my argument stands.
You say adaptable and receptive. I say receptive and vulnerable. And let's take a look a few paragraphs up. "Thrallherds manipulate the minds of others as if they were clay in the hands of a sculptor." As I said, a pliable object needs someone to ply it. When thinking of others' minds in terms such as this, with a few exceptions, is a sign of superiority, arrogance, and corruption. But wait, there's more.
"Some creatures are more suceptible than others to the thrallherd's unconcious, but continual, call to service." The unconcious line is about the only save against evil they have, but unless your backstory has it that she doesn't know that she's an enslaving hellbitch that doesn't really address the point at hand. Moving on.
"And so they come, eager to be led, happy to follow, and completely under the thrallherd's control." Really, if that isn't a setup for "power corrupts" I don't know what is.
"In this way, thrallherds keep a minor menagerie of enthralled servants that are anxious to do their will."
If you are seriously going to defend that the tone of this is ethically neutral, I'm just going to call you quaintly naive right here and point you down another couple paragraphs to where the only example of thrallherd behavior in the entire writeup involves sacrificing believers without regard for their lives because the class abilities are geared to make that practical behavior.