[jewbacca]

> That would make the whole downvote system useless

This would not remotely "make the whole downvote system useless". The primary use of the voting system here is to sort out discussion by subjective value. That would only be very indirectly affected (through the very distantly secondary use of karma to confer downvoting ability).

If anything, it would debatably enhance the primary use of the downvote system, by giving authors of unpopular comments an incentive to remove from the discussion what the community has judged to be noise.

Though I would personally hate that, as a fundamentalist anti-deletionist, and as a frequent dissenter from this community's judgements of value.