With regard to the mentioned thread being hijacked, Julia and I had a short private convo about this, and she proposed the idea that forum sages should get pruning power, and I agree.
I imagined that sages would see another reply button aka “prune reply” which does what the “Reply in New Topic” does, but also takes the original off-topic post being replied to, along as the OP of the new thread.
This would need to be proposed over at Discourse Meta.