I know that might sound trite, and it's obviously not the only factor, but I bet if karma went away, you would see a measurable decline in submissions over time.
The karma of the submitter has no effect on how a story is ranked. Nor I suspect does it affect whether people upvote the story, except when the submitter is one of maybe 100 HN users who are well enough known that their username as submitter makes people pay attention to the story. Plus if we noticed anyone consistently rewriting titles we'd ask them to stop and eventually ban them if they didn't.
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