Culture. Those unhappy with Twitter are unhappy primarily because of the culture, not the functionality (mostly). And it seems that the Threads users who are enjoying the platform are enjoying it for two main reasons: 1. organic engagement (which, to be fair, will plummet in short order) and 2. a less contentious culture.

Essentially, it could succeed simply because it appeals to a different, albeit overlapping, audience: the Twitter-haters and Instagrammers who never made the jump to Twitter but whose interest is piqued by a different type of social media (different from IG, that is).

You're right to point out that being a Twitter alternative in and of itself is not enough, but there is a major difference here: By linking it to IG, they removed almost all of the friction normally associated with signing up, hence the 100+ million.

I don't think it's a "Twitter-killer," but I suspect it's got staying power. I think they'll coexist, serving different audiences.