Archiving doesn’t really suit our needs. In the case of duplicate threads, we don’t want them showing up in the search (hard to find the one, true thread when there are a bunch of archived duplicates polluting results). Since they’re fully searchable, it’s also still possible for users to dig up dirt e.g. “haha look at this bad thread you posted a year ago”. Unlisting or deleting are preferable by far in these cases.
As for deleting, we still want the OP to be able to go back into their thread with the link we send in feedback PMs for reasons mentioned in the OP. Unlisting seems like the best option for us.