[the_af]

Agreed. But like email, I expect forum threads/posts to stay indefinitely and be searchable, until I explicitly delete them.

Email is a persistent conversation which gets archived indefinitely, not an ephemeral one like talking with our mouths.

[nine_k]

Your personal email may be retained indefinitely.

Corporate email (and IM, etc) gets purged pretty quickly, within 2-3 years, unless you save a particular message / thread explicitly.

[BlueTemplar]

Aren't they worried about losing most of the knowledge of the context of decisions made?