What we actually care about is having as much high quality content that has as many readers as possible.

I would argue the median high quality comment is already made by a 1000+ user. In any case the limit is something we can easily change based on experience and isn't something that should be set without at least first seeing a graph of karma distribution among users.

There's probably a corollary to Löb's theorem that says a community of rationalists can't add new members to the community and guarantee that it remains a rational community indefinitely. Karma from ratings is probably an especially poor way to indicate a judgement of rationality because it's also used to signal interest in humor (to the point that slashdot doesn't even grant karma for Funny moderations), eloquence, storytelling, and other non-rational things. Any karma-increasing behavior will be reinforced and gain even more karma, and the most efficient ways of obtaining karma will prosper contrary to the goal of creating high quality content. Does every user with more than 1000 karma understand that concept sufficiently to never allow a user who does not understand it to reach 1000 karma?

To be honest I didn't fully grasp the concept until just now. I was ready to start talking economics with karma as the currency until I realized that economics can not solve the problem.