I don’t think that it materially changes the debate whether the topics were originally imported to Discourse or usser created in Discourse.
The desire to remove clutter and tidy up the forum does not have to invoke merging of topics. Archiving a topic leaves preserves the discussion better than merging but ensures that redundant topics don’t take up space in the active discussion space: on the top page, in the random topics at the bottom of the page and in the digest emails.
The case for merging user accounts has far better justifications. Particularly where an existing Discourse user has more than one email account or where an imported user had multiple accounts in another product. Such changes are far less likely to materially impact topics because it is much less likely that they would be using both accounts in the same topic.
Merging user accounts should not affect the two big weaknesses of merging topics which is either or both of the impact on the chronology of posts and the resequencing of topic posts.