The big issue is lack of incentive. Stackers don't get any reward for zapping old content, which means they don't put any effort into finding it and engaging with it.

I think there are some ways to fix this and have stuff recirculate through the hot and top feeds.

Undisciplined

What is the worst thing about SN?

That's what the "Random"-list is for, no? There are also some users who've made it their Jam to curate and essentially wrap said posts in a fresh post. 

I think this one's not something SN can "fix" per se, but something we'd have to make a conscious decision about on an individual / Community-Level.