If we were to update the list automatically the page would be a total mess on a busy forum as topics would have to move around every time a new post comes in.
You do have a point there.
However, you could still do it in a Facebook-esque style:
When a reply is made to an existing topic (or a status, on FB) is made, do nothing (or alternatively, only update the reply count & "X posted Y minutes ago" snippet)
When a new topic is made, add it to the top of the list
List every topic in order of most recent reply (current behaviour)
But that's what pagination is for. And if after 5000 posts the subject of the original topic has not been resolved, then it will never be.
I have a tutorial on my blog with more than 550 comments. If I had a good forum platform with a good search feature, most of the comments would not have been asked.
This is related to an issue resolved earlier.
We found that if a topic's posts contained images pointing to a relative path (e.g. ![test](1.jpg), NodeBB would attempt to load them, causing them to load the entire topic again (since the url is site.com/topic/123/1.jpg).
To combat this, @baris restricted the URL so that the slug must be correct, otherwise the user is sent to 404.
For lack of a better solution, this was the approach we took, so this seems to be the reason why you're running into this issue now.