Yes, it's a good suggestion. It will work well for topics that are not valid after a certain period of time. But there are some topics which might be interesting and are something that can be valid even after an year. So they can be left open.
It is a good suggestion and prudent too for many threads that that run out of steam in a couple of weeks. But there are also threads that can provide good discussion even after a couple of years or a few months. As and when possible, the moderators do lock such thread but keeping a track of every single thread that exists is a very difficult task seeing that everyday new threads are constantly being created. Also time is an imposing factor are all moderators are part timers and work at other jobs. The most helpful thing here to bring such threads to attention of moderators and requesting them to be locked can be done by members as and when you come across them. Just use the Report button on the thread and put note on why you have done so. That would help bring such threads to notice and can then be locked.