If Alice makes a followers-only post, and Bob replies to it, to whom should Bob's reply be visible?
@evan You asked "should", not "does", so my answer is "only people who follow *both* Alice and Bob", that is, the intersection of both sets of followers. (Your 3rd option, to me, reads like "people who follow *either*", that is, the union.)
Whether Alice-only follower Carol (resp. Bob-only follower Dave) should gain access to Bob's reply by following Bob (resp. Alice) after Bob's reply is unclear to me.
@pauamma so, the audience for replies gets smaller and smaller as the conversation grows?
@evan Good question. I'd have to think about it more, but my hot take is "it depends a lot on context". (Consider for instance that Bob may be Alice's stalkery ex.)