You can't trust the client. It's the same fundamental flaw with DRM. If that person does not want it archived, you can make it difficult but not impossible.

I don't think it's nearly as difficult as DRM. That feature is mostly a gimmick. A lot like Snapchat's.

This might help with security if it takes as long to crack someone's phone as it takes to expire the messages.

Actually, I can get the source code for the client, delete the "auto expire" feature and compile it. So there's nothing stopping anyone from archiving messages you want them to be able to read.