> that's probably why 4chan has a Perma-age function in the first place.

You mean the option to "sticky" threads? I always found that a pretty awkward feature with which mods impose content on the users and it also goes against the general philosophy of imgboards in which threads are not really supposed to last for long. But much more than that, I find it really annoying to have one or as much as five threads constantly at the top - yknow why I sage most of the time? Because I don't want to annoy others by bumping a thread up with useless or boring replies - which most stickies on 4chan consist of.

> I find that the oldest threads on 2ch boards only date back as far as about mid-2004, so the rest must have either been deleted or archived.

They get archived. I haven't figured out yet how this works, but some are publically accessible but for the majority you have to buy an account ($30/year) to access them.

> why should a poster be able to exempt himself from being identified with an ID code simply by saging a thread?

Let's propose that question in a different way: If IDs are not supposed to be forced on a board (for which several good reasons exist but I haven't seen too many western boards out there who do this) why should a user be exempt from taking advantage of this option?
The only reason that came to my mind was: by forcing him to not bump a thread with his replies - replies which, taken into consideration that he wanted to make them without ID code enabled - are more likely silly, boring, annoying or flames or whatever.