I don't think that the karma decay function would work or be helpful. Most people on the leader board, got there from hard work and dedication to make this site a really cool place.
I know that my participation in this board was a deliberate exercise in community building. I worked my tail off in trying to find interesting material that my fellow hackers would enjoy reading. It's pretty common for me to spend 20-40 minutes crafting a response to a comment.
A number of us were upset at the downfall of Reddit, and we were determined to make this a better place. I was one of those. And, while I still use a highly curated form of Reddit for news. I never participate in discussion there.
So the leader' karma scores reflect how much we've invested in this site. I don't think that's a bad thing. People on the leader board have made this site what it is. And, they're still the backbone of the site. Can you imagine what it would be like if nickb or edw519 stopped submitting? It would dramatically change the tone of the site, since their contributions to it are so substantial.
I know that I haven't been as active the past few months, but that's because school's done for now, and I've been building product. With any luck, I'll be launching by January. We'll see.
But adding a decay function to a site like this would only punish people who have contributed lots.
Maybe a better answer would be to increase the size of the leader board to the top 200-300 karma. That would give some recognition to the people that are putting in more work on the site, gut their karma isn't on the top 100. Just a thought.