[spindritf]

> If you can point me to any web discussion communities that are 10+ years old which use threading, I'd love to see them.

There are virtually no such communities because 10+ years ago people were still having serious conversations on mailing lists and USENET. Those were threaded and had specialized clients allowing to easily find new posts, replies to own posts, filter content, etc.

The great improvement web brought wasn't flatness but post-moderation, editing and later karma/voting/sorting. You simply couldn't moderate articles after they were posted on USENET. And editing ("superseding"?) mostly caused confusion over what your readers are seeing depending on the server and software they're using.

[saurik]

(I largely agree with you, but will still point out that Slashdot is 10+, 15 in fact, and was/is an explicitly arbitrarily-deep threaded discussion system.)