You know it really shouldn't be a standard. I understand the not wanting of bumping old threads, but this forum is so freaking slow and not a huge community. I just noticed this when I went to bump my Fable Discussion thread. That thread had some nice discussion in it from before, I wanted to keep the same thread because of the history, but no I couldn't and it made me sad.
So instead I just copy and pasted the entire first post to make a new thread. I never really saw the bumping here as an issue. If a newb bumps a thread from a user no longer around and you don't want that thread bumped you could just lock it. But in the case of threads that lay dormant because the topic itself hasn't had discussion in awhile and the user is still active then we should be able to bump our own old threads at the very least.
My suggestion, don't make it a forced auto archive. Just enforce the rule yourself as you see fit as a staff. It's not that much effort.