Just some clarification here.
1. Is the expiry date you were talking about is activated by default or OP need to put an expiration date for the post/thread?
2. Expiration date for posts?
Should there be an expiry date for posts, after which the are locked or archived?
Do you mean thread? Because posts can't be locked.
If yes then it's a bad idea. Just imagine, let's assume that there is a expiration date for a thread then how about those threads that were useful and having an ongoing discussions like this one
Merit & new rank requirements? Then it will be locked after a certain period of time even though there are people discussing something. Hmm bad idea.
Scrolling through there are so many posts with repeated replies, and some other posts which are giving wrong information
The best thing you can do to get rid of posts like these is to hit the "Report to Moderator" button in those posts which are useless that you were talking about.