Part of the problem seems to be that the "similar threads" feature is matching up really ancient threads that really do not need to be dredged up from the past. We're actually trying to work on closing old threads that are no longer consistent without current guidelines (part of this is the reason many resurrected discussions get closed recently). We used to have much more leniency about crackpottery and unsubstantiated speculation in threads. And, quite frankly, we used to be pretty bad about letting people toy with problematic posters rather than just delete the posts and ban the trouble-makers. None of that needs to be dragged out of the past. As we find those, we lock them to make it clear they are no longer consistent with current guidelines.
Sometimes, it simply makes no sense to drag up an old thread, when someone was asking for advice on something that is completely irrelevant by now. Just for example, I saw one in academic and career guidance dredged up from 4+ years ago started by someone asking about college choices. The question was very specific to the OP and the person resurrecting it was responding to that question, not in a general way that would help others in similar situations. There is no point in dragging up a thread like that...the OP has long ago made their decision, and is likely already graduated from college.
Likewise, there are old posts that were harmless at the time, mostly jokes. When posted, this was clear because the general context of the member's other posts at the time or other posts in the forum at the time indicated the proper tone. Sometimes these get resurrected by someone not appreciating the humorous intent, or where the original context is completely lost. Again, these should not continue in the original thread.
Some topics can safely be revisited. For example, someone may have posted a while ago about "new" research on something, and if there is now a follow-up study, it would be appropriate to update the original thread if everything else in that thread conforms to current guidelines.
humanino's suggestion is a good one as well. If an old discussion triggers a new thought, a link to the old discussion for context while starting a new thread may be a good option as well.