People do that in games all the time. Finish the game -> restart

As long as the ride is good, people don't mind having to step down and queue again

Same way term limits make sense. Hit the ceiling, then you're done. Make room for someone else. Avoid stagnation.

You get paid in power, money, fame, access ect for being president. Providing content for free to make someone else money doesn't have the same appeal that could tolerate term limits.

If you can simply register another account and start over, how would this help avoid stagnation?

You'd lose any privileges associated with a high number of karma points, also all of your previous content would be removed, so people could cover the same territory in possibly new ways with less fear of being called redundant.

It basically compensates for first-mover advantage in forums.

As a thought experiment, your idea is interesting!

I see a few problems though. This wouldn't work for just any karma-based site. For example, on StackOverflow where there are good "canonical" answers to tech problems, and these get heavily upvoted (because they are useful), having them removed and later rephrased (possibly incorrectly) by a new user would be detrimental. So any system where "good" posts are archived and referenced wouldn't benefit from this.

Even when there are no canonical/permanent answers, the thought that every "good" post you make takes you nearer to account deletion is not exactly enticing. Why not simply get rid of points and make all posts ephemeral, kind of like Instagram stories? What good do points even do in a system where your account and posts will get inevitably deleted?

Also, wouldn't mediocre/bad accounts stick longer than "good" accounts, making the whole system worse? I don't mean terrible or troll accounts -- I understand the point system would still be used to ban or hide them -- but mediocre accounts which post mediocre/bad posts. Wouldn't they dominate in this kind of system?

If upvoting a post/comment (which gives the author karma points) would risk triggering its deletion (by making the author cross the karma threshold), then is that not effectively turning the karma system on its head?

Under such a system, if I think a post is valuable, I would then try to "protect" it and its author's account by downvoting it. Similarly, if I think a post is not valuable, I can help trigger its eventual deletion by upvoting it.

That would only really matter if the poster were really close to the threshold. Moreover the gaming of the system would be unlikely to work forever, requiring too much coordination of effort.

But to answer your question, yes, it is turning the karma system on its head for people near the top of the standings; the idea being that having "titans of karma" in the community becomes detrimental at some point.