On the other hand, your description bears a strong resemblance to Twitter. If you have a conversation with someone through replies, anyone who follows both of you will see it in their timeline, and by default,
anyone can read it by going to either of your profiles. So it’s not uncommon to have someone you’ve never heard of jump into a conversation. And it’s easy enough to “snoop around” yourself; sometimes that can be a source of genuine new acquaintances, and with time, friendships. (There’s an etiquette to it, though. If you don’t know someone, you should only butt in to their conversations if you have something to add - not because, say, you disagree with a political statement they’re making, or at the very least, certainly not if you’re planning to hurl insults!)
Unfortunately, some people and some topics tend to attract a lot of those sorts of uninvited rebuttals - in other words, harassment - driving some to make their accounts private. In that case, only approved followers can see your tweets, ever. There’s no “friends of friends”-like middle ground, or any other more flexible filtering options.
On the other hand, for most people, Twitter’s system seems to work pretty well. It works for me, and I really enjoy the spontaneity of it.