"Talk page are used for other purposes in addition to sending messages to a person, or joining in a discussion."
This first point is my primary reason for arguing that the "Board" remain distinct from the "Feed". The fact that Boards will be used as "quick glances" into the life of an editor is super-important to me. We have been discussing what other things might need to appear on the board in addition to discussions. Block noticies? If they are an administrator, should we indicate that they have performed administrative actions (e.g., "Bob blocked Vandal23342")? What about main space contributions?
These are the kinds of questions that we're wanting answers to.
(And, by the way, it is currently planned that deleted posts and topics will remain on the board, just noting that the topic was deleted and by whom. So you won't have to check the history to see that this has happened.)
"There are also some people whose talk p. is watched by a very large number of people--sometimes several hundred , most of whom do not really plan on posting there, but want to see what is being discussed there, under the assumption that some of what they find out about may be interesting."
This is exact and precise use-case for being able to subscribe to other users' boards. The Feed/Subscription model will actually make doing this easier, not harder.
The end vision of Flow is not to restrict information flow - far from it. And, in fact, the entire point of the project (from my mind) is to actually increase project-wide awareness.
Right now, if you want to see what's going on with the seven WikiProjects you're a member of, you have to go to seven different pages and see if they've been updated (or view diffs from your watchlist, which is painful). Think about a future point where new posts and new requests on those seven boards automatically flow into your feed. This increases collaboration power.