Well, Elon definitely doesn't.
> the fact that they're heavily branding it with Instagram, using Instagram logins etc suggests to me that they're just looking for another angle to vacuum up user data
They're using the most popular social network which they happen to own, which already has pre-built social connections for most people who might want to try Threads. Almost none of my real-life friends are on Twitter, most of them have active Instagram accounts.
> when was the last time Facebook successfully launched a new standalone social app
It doesn't really matter, what pays off is being able to run experiments faster. Also, despite all of this they don't have a reputation for killing working products. They either dead before this or become good. Applying the past experience doesn't necessarily provides a good estimate for Threads's future.