The silly rule is a bit like "No loc" rule that got revised earlier it doesnt take anything into account its just a old thread that get topped for some reason. Mods/admis doesnt take into account anything else. Its a rule that isnt stated anywhere and it creates a lot of confusion. Most people have the approach that if there is already a thread about the subject they want to discuss they look for a old thread to post their question in there. Then mod/admin comes and close the thread "No necro" no additional information. ITs a bad approach, how are people supose to know that they are supose to create a new thread rather than use a old existing thread? (A bit like how are we supose to know when "off topic rule" is applied).
So either make it clear that necros arent prefered and you rather want them to create a new thread or just remove the rule at all. It doesnt really server any purpose either way.