Auto lock threads? Or...profile comments? That seems more worthwhile. Why reply many times when you know that person is gone?
No, they would do everything else they normally do, but if threads auto locked, people wouldn't go back and bring old threads back.
There is a difference between reviving and necroing. Reviving is trying as much as you can to save the life of a thread that's still holding on, while necroing is bringing a dead thread back to life with primitive methods.
While threads do go on in months, reviving them is totally legit, within reason. Sometimes making a new thread to cover the old one with many replies to it is just a waste of infinite space.