I honestly think this is a very silly take. I don't think any of this is how any of this works.
Threads attracted two kinds of people: 1. People coming mostly from instagram / tiktok who had never considered using Twitter because it had an established reputation and they knew it wasn't the kind of product for them, and 2. People switching from Twitter because they love Twitter-the-idea but can no longer stand Twitter-the-actual-product-today.
Most of people from group #1 tried it and concluded that yep, it wasn't for them. There is zero surprising about that, and it's where the giant initial numbers came from. But if any of those people stuck around, that's pure bonus.
The more interesting question is what's going on with group #2. Certainly lots of them decided there were too many missing features at launch and kept using Twitter mostly. But that is not a "they'll never check back", those people are still in play for any of Twitter to keep or its competitors to win eventually. But the current DAUs wouldn't be where they are if a significant portion of this group hadn't decided to actually stick around. And that's pretty surprising.
For years the conventional wisdom has been that you can't actually convert users from one social network to another in the exact same niche. You can cut off growth - like instagram adding stories corresponding to Snapchat's growth plateau - but people stay where their existing networks are.
But not this time!