We appreciate all the feedback, and we've made some adjustments, like excluding BST threads from the "similar threads".
We've heard the arguments for and against this new system, and based on our findings, the pros far outweigh the cons. That being the case, for now we're not planning on making any other changes to the threads shown in this section.
Regarding auto-locking old (or any) threads: This is something I personally feel very strongly against, and it will probably never happen on any of the forums I own / manage.
Thanks again for all the feedback!