Automatic locking of threads is a possibility. I've discussed this with our volunteer moderators and they've expressed some interest. It may end up being a per-forum section application.
I hope you don't go that route, it's a double edged sword.
One of the largest online forums -
trip advisor has this feature and it sucks. I can't tell you the amount of times I do a search for something on google for a travel subject, I get a trip advisor thread in the results and click on it and find the perfect match to what I'm asking about, read through the thread and see I need to post a follow up only to discover the
"This topic has been closed to new posts due to inactivity".
Then you have to start the whole process over instead of simply adding a question relevant to where the discussion left off.
Yes, Zombie threads happen, but so what? If the old thread is relevant enough for someone to revive it what difference does it make?
Forums are about discussions and activity, anything you do such as locking old threads is going in the wrong direction when your goal is to promote activity.