Just my opinion but a 'long dead' thread could have offered some advice to do a certain thing which, a couple of years ago, may have been the safest way to get round a problem.
In the intervening years, it may have been discovered that the procedure that was advised is now either against the regulations or positively dangerous.
To post a correction could possibly save someone who has a similar problem and is searching for a solution, from trying out a deadly or dangerous cure to their problem. If they came acxross the 'long dead' thread and read through it they would discover a more upto date solution.
Don't lock old threads for that reason, just monitor who and what is being posted in resurrected threads.