The better question to ask is, where do threads go after
expiry? Thread heaven and not even g/mods can bring them back to
earth.
Depending on your definition of archive, to some it is total
removal of the thread and postings out from the forum and onto tape
or even hard disk. The file size are usually compressed or zipped
to save disk space and tapes. For some it is just a lock to prevent
follow ups and bumping.
So as years go by, the forum would have lotsa of tapes or disk
backup and they cost money to maintain. So the cost of hosting a
server will need to factor in this cost too 
