Honestly, I just think this is another feature which wouldn't benefit the forum too much. If threads are becoming redundant, and encouraging spam you can report them.
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?
I doubt anyone would be willing to put an expirty date on their thread. For one, they don't know when they are going to get an answer, and it might take several weeks for them to get a reply which actually answers their question etc.
But, also users don't lock their own thread now. I don't think they would care for automatically expiring their thread.