I understand what you mean about dead links, they are annoying and should be fixed somehow when found if possible. But even a thread or post with a dead link can be useful so we will need to be cautious if we were to delete.

Even threads with zero replies can be useful so I do not think a blanket approach would work.

As for the thread you linked in the above post, I do not see a problem with it. It explains what happened to a valued member and also gives a way to obtain some of the information from his old website.

users could at least rise a check-flag (mark) when it is obvious that a thread is not helpfull, it would be a bit of a start.
Not helpful to who?
Currently there is a way to rate a thread, maybe if users used that option more? I am not sure if that option is tied to the search function. We will have to play with this.