Yeah, that's the one scenario that makes sense to me. If you're already deep in the IG ecosystem and what-not, then maybe it makes sense to jump on Threads. I wonder if they'll be able to attract many people who weren't already IG users though?
Then again, while I know IG is popular, I don't know how many actual users they have. Maybe "everybody in the world" was on IG and will wind up on Threads now as well? shrug
They like that they have a bunch of people following and reacting to their posts already, since they got ported from Instagram.