@tinyrabbit I can see why you would want that, & I'm sure I'm not going to say anything you haven't thought of already, but my thinking is this.
Suppose there's already a single thread, not too long, easy to read perhaps 10 or 20 toots. But one of the toots in the middle is something to which I'd like to reply. The existing system allows me to do that, whereas having a single thread would force me to tag my comment on the end, even though I'm replying to something further up.
CC: @humanetech