I wouldn't say kbin would be harmed necessarily even if maybe just a tiny bit. A good amount of the users on Lemmy (can't say much about kbin hadn't tried it yet) hate threads. So as an alternative to Mastodon, that would actually more likely score them points not detract from a user perspective.
Most likely, for a user to be on kbin or Lemmy you either come from Reddit, or you come from Mastodon so the users would had likely made up their mind on separating themselfs from major platforms, that likely includes threads.