@evan I think there should be 2 settings: "followers only" and "followers cascade" (or something).
The first restricts it to Alice's followers only. So Bob's reply is not visible to any of his followers that are not also Alice's followers.
The second is visible only to Alice's followers when posted but becomes visible to all Bob's followers once he replies.
This second setting would probably more safe than a general public post, based on the birds of a feather hypothesis, but less safe than the first.
I an ideal world, where everyone behaves themselves, all posts should be public for all. I'm going to take a nap now until that happens. Wake me up when it comes.