No necroing fragments the dicussion since when you land from google on a thread you can't respond and it's a lot of work to set up the same discussion context as in the thread you planned to reply on - I wouldn't bother. I post threads where the most interesting replies usually come within few years from people landing in the thread from search.
Ideally, I'd like if I could have anonymous, no registration replies in my threads. My threads should be like a hybrid of wikis and forums, and if I reference something with a link, that would create a copy of the linked content, in case the linked content goes down years later, it will still be available automatically.
If someone tries to answer a old thread, have them answer a question which I as a thread poster made up (or captcha as alternative) - then if they are correct, their post shows up for them and me but not others. If I approve the reply then the thread opens for posting as normally.