First I will say that I wouldn't lose any sleep over it. The reason being how can you miss someone you don't even know is there in the first place??;)
To me the lurker issue is more about changing peoples minds as opposed to punishing people who don't contribute. If we set up a rule like that it would only force people who don't want to contribute (even with comments) to make insincere replies just to stay here.
I have had some small results over the years by constantly harping about this issue. It makes me feel good when someone I have been after to contribute something finally starts replying to other people's posts. I also think that those people who finally did find out that it's much more fun to be involved and make friends here.
As far as people who haven't even been on the forum for more than a year, I could see deleting their accounts if it would benefit the forum somehow, if it wouldn't then it doesn't really matter either way.
One other thing, I was a lurker when I first got here. I was new to the forum idea in general and really didn't have much to say. Then I saw some posts, wanted to see more and started asking questions...that is how it starts. Now years later I couldn't imagine being here and not contributing at the very least replies to the people who take time to post things people enjoy.
It's one thing to be shy, but seriously this is as anonymous as it gets. Nobody knows you or can see you. Say something, post something, get involved and you will have a better forum experience guaranteed.:cool: