It’s entirely possible that Threads never succeeds to the level of Twitter, but still sucks up enough eyeballs and revenue to make Twitter unworkable as a business.
Twitter has a large user base who just use it because it has 500M daily active users.
All it takes is for a solid chunk of the high-revenue "boring mainstream" audience to exit in order for Twitter to be permanently damaged as a revenue-generating enterprise, even if it still has lots of users. (And as a Twitter addict I think that would be sad! Not sure why the downvotes over a legitimate concern.)