There really should be some karma decay system to prevent early users hogging 5-figure reputation. I didn't do much there in the last 5y but I am still way up in reputation.
I don't see how you can call it "hogging". Having a high reputation doesn't prevent anyone else from getting a high reputation too.
For me the reputation points are a proxy for how helpful I've been to other people overall. I like being helpful. Suddenly seeing that score go down for absolutely no reason would be very disincentivizing for me.
Gradual 5 year drop-off for effective karma might do community good. As such any post older than that would have zero effective karma, but you could still show historic karma. Maybe this would make moderation better for new users as old hostile ones lose their power if they don't keep participating in the same pit as rest new userbase...
I think this idea combined with the “is this answer still relevant” flag that was recently added would work well. I see no reason to devalue old but still good answers.
Sometimes you're forced to work with older technology, and it's nice that you can still find answers to questions that many would consider obsolete. I'd hate to see a clean-up effort that makes that information go away forever.
For me the reputation points are a proxy for how helpful I've been to other people overall. I like being helpful. Suddenly seeing that score go down for absolutely no reason would be very disincentivizing for me.