A plugin might resolve your technology problem though: don’t allow post add/modify in archived topics. It could prove the popularity of your preferences. If it become popular then maybe the changes you want could be incorprated into Discourse.
The visual is an incomplete representation. I prefer to look at the actual functionality rather than metaphors.
I said changing status from archived to unarchived which is a larger task than changing the archived item itself.
Not true. Discourse archiving substantially removes the topic from the user interface. The main path to find some topics is then explicit links. Archiving is like a partial unlisting to reduce casual interactions:
Unlisting completes the delisting process which is why it is relevant.
Technically, most archives on the Web are “snapshots” of the presentation layer. That is also why they are usually read-only because they do not incorporate the entire entity they represent. They are more like photographs, as snapshot suggests. They usually lack the program code and database structure that are necessary to allow further interaction with the original entity.
Looking more specifically at forum archives (and blogs, email, and other media types), there are varying levels of interaction available. All archives are not read-only and many can be unarchived simply by clicking restore/publish. The further the archive is from the core forum code and data structure then the more read-only it usually becomes. I know that some forums have problems unarchiving where the original categories no longer exist. Discourse archiving avoids that problem.