First of all, I doubt the premise of this entirely, e.g. Pokémon Go almost certainly had higher DAU weeks after it launched (and it was growing rather than shrinking).

But secondly, I'm skeptical that it's even fair to talk about Threads stats using the same measurements as any "new" app, since it's really an extension of Instagram. You don't make a new account for Threads, you just use your Instagram account. They are one and the same, the only difference being which app you use to access which features. It's most closely analogous to Facebook and the Messenger app - do you count the users of each of those separately?